Remove window management logic

Since everything now happens in a single window, there is no need
to have a way to keep track other windows. They simply won't exist.

But on windows and Linux it will still be possible to start
multiple windows by starting various Dangerzone processes. On MacOS
this doesn't seem to be as easy from the launcher, but it should
not be critical as multiple documents can be converted at the same
time in the one window.

dangerzone/gui/__init__.py

@@ -20,7 +20,6 @@ from .main_window import MainWindow
 
 class Application(QtWidgets.QApplication):
     document_selected = QtCore.Signal(str)
-    new_window = QtCore.Signal()
     application_activated = QtCore.Signal()
 
     def __init__(self) -> None:
@@ -80,32 +79,20 @@ def gui_main(filenames: Optional[List[str]]) -> bool:
     # Allow Ctrl-C to smoothly quit the program instead of throwing an exception
     signal.signal(signal.SIGINT, signal.SIG_DFL)
 
-    closed_windows: Dict[str, MainWindow] = {}
-    windows: Dict[str, MainWindow] = {}
-
-    def delete_window(window_id: str) -> None:
-        closed_windows[window_id] = windows[window_id]
-        del windows[window_id]
-
-    # Open a document in a window
-    def new_window(filenames: Optional[List[str]] = []) -> None:
-        window_id = uuid.uuid4().hex
-        window = MainWindow(dangerzone, window_id)
+    def new_window(filenames: Optional[List[str]] = []) -> MainWindow:
+        window = MainWindow(dangerzone)
         if filenames:
             window.content_widget.doc_selection_widget.document_selected.emit(filenames)
-        window.delete_window.connect(delete_window)
-        windows[window_id] = window
+        return window
 
-    new_window(filenames)
+    window = new_window(filenames)
 
-    # Open a new window, if all windows are closed
+    # MacOS: Open a new window, if all windows are closed
     def application_activated() -> None:
-        if len(windows) == 0:
-            new_window()
+        window.show()
 
     # If we get a file open event, open it
     app.document_selected.connect(new_window)
-    app.new_window.connect(new_window)
 
     # If the application is activated and all windows are closed, open a new one
     app.application_activated.connect(application_activated)

dangerzone/gui/main_window.py

@@ -20,12 +20,9 @@ log = logging.getLogger(__name__)
 
 
 class MainWindow(QtWidgets.QMainWindow):
-    delete_window = QtCore.Signal(str)
-
-    def __init__(self, dangerzone: DangerzoneGui, window_id: str) -> None:
+    def __init__(self, dangerzone: DangerzoneGui) -> None:
         super(MainWindow, self).__init__()
         self.dangerzone = dangerzone
-        self.window_id = window_id
 
         self.setWindowTitle("Dangerzone")
         self.setWindowIcon(self.dangerzone.get_window_icon())
@@ -82,7 +79,6 @@ class MainWindow(QtWidgets.QMainWindow):
 
     def closeEvent(self, e: QtGui.QCloseEvent) -> None:
         e.accept()
-        self.delete_window.emit(self.window_id)
 
         if platform.system() != "Darwin":
             # in MacOS applications only quit when the user