Add a Qt widget for creating collapsible sections

Add a Qt widget called "CollapsibleBox", in order to build sections that
you can hide/show with a single click. There is no native widget for
this functionality, so we borrow some code from a StackOverflow user:
https://stackoverflow.com/a/52617714

dangerzone/gui/logic.py

@@ -216,3 +216,93 @@ class Alert(Dialog):
         message_layout.addWidget(label, stretch=1)
 
         return message_layout
+
+
+class CollapsibleBox(QtWidgets.QWidget):
+    """Create a widget that can show/hide its contents when you click on it.
+
+    The credits for this code go to eyllanesc's answer in StackOverflow:
+    https://stackoverflow.com/a/52617714. We have made the following improvements:
+
+    1. Adapt the code to PySide.
+    2. Resize the window once the box uncollapses.
+    3. Add type hints.
+    """
+
+    def __init__(self, title: str, parent: Optional[QtWidgets.QWidget] = None):
+        super(CollapsibleBox, self).__init__(parent)
+        self.toggle_button = QtWidgets.QToolButton(
+            text=title,
+            checkable=True,
+            checked=False,
+        )
+        self.toggle_button.setStyleSheet("QToolButton { border: none; }")
+        self.toggle_button.setToolButtonStyle(QtCore.Qt.ToolButtonTextBesideIcon)
+        self.toggle_button.setArrowType(QtCore.Qt.RightArrow)
+        self.toggle_button.clicked.connect(self.on_click)
+
+        self.toggle_animation = QtCore.QParallelAnimationGroup(self)
+
+        self.content_area = QtWidgets.QScrollArea(maximumHeight=0, minimumHeight=0)
+        self.content_area.setSizePolicy(
+            QtWidgets.QSizePolicy.Expanding, QtWidgets.QSizePolicy.Fixed
+        )
+        self.content_area.setFrameShape(QtWidgets.QFrame.NoFrame)
+
+        lay = QtWidgets.QVBoxLayout(self)
+        lay.setSpacing(0)
+        lay.setContentsMargins(0, 0, 0, 0)
+        lay.addWidget(self.toggle_button)
+        lay.addWidget(self.content_area)
+
+        self.toggle_animation.addAnimation(
+            QtCore.QPropertyAnimation(self, b"minimumHeight")
+        )
+        self.toggle_animation.addAnimation(
+            QtCore.QPropertyAnimation(self, b"maximumHeight")
+        )
+        self.toggle_animation.addAnimation(
+            QtCore.QPropertyAnimation(self.content_area, b"maximumHeight")
+        )
+
+        self.toggle_animation.finished.connect(self.on_animation_finished)
+
+    def on_click(self) -> None:
+        checked = self.toggle_button.isChecked()
+        self.toggle_button.setArrowType(
+            QtCore.Qt.DownArrow if checked else QtCore.Qt.RightArrow
+        )
+        self.toggle_animation.setDirection(
+            QtCore.QAbstractAnimation.Forward
+            if checked
+            else QtCore.QAbstractAnimation.Backward
+        )
+        self.toggle_animation.start()
+
+    def on_animation_finished(self) -> None:
+        if not self.toggle_button.isChecked():
+            content_height = self.content_area.layout().sizeHint().height()
+            parent = self.parent()
+            assert isinstance(parent, QtWidgets.QWidget)
+            parent.resize(parent.width(), parent.height() - content_height)
+
+    def setContentLayout(self, layout: QtWidgets.QBoxLayout) -> None:
+        lay = self.content_area.layout()
+        del lay
+        self.content_area.setLayout(layout)
+        collapsed_height = self.sizeHint().height() - self.content_area.maximumHeight()
+        content_height = layout.sizeHint().height()
+        for i in range(self.toggle_animation.animationCount()):
+            animation = self.toggle_animation.animationAt(i)
+            assert isinstance(animation, QtCore.QPropertyAnimation)
+            animation.setDuration(60)
+            animation.setStartValue(collapsed_height)
+            animation.setEndValue(collapsed_height + content_height)
+
+        content_animation = self.toggle_animation.animationAt(
+            self.toggle_animation.animationCount() - 1
+        )
+        assert isinstance(content_animation, QtCore.QPropertyAnimation)
+        content_animation.setDuration(60)
+        content_animation.setStartValue(0)
+        content_animation.setEndValue(content_height)