Fix Settings().set() when setting new setting

Settings().set() would fail if we were trying to set a setting that did
not exist before. The reason is because before setting it would try to
get the previous value, but though direct key access, which would lead
to an exception.

dangerzone/settings.py

@@ -47,7 +47,10 @@ class Settings:
         return self.settings[key]
 
     def set(self, key: str, val: Any, autosave: bool = False) -> None:
-        old_val = self.get(key)
+        try:
+            old_val = self.get(key)
+        except KeyError:
+            old_val = None
         self.settings[key] = val
         if autosave and val != old_val:
             self.save()