qt/qml: fix shortcut dialog handling

• Allowing preferences dialog to be opened multiple time freezes the application. I think co-existence should not be allowed. I did not change it to a singleton because it seems it should not allocate memory when not shown.
• Various methods in DialogsProvider use static methods that are available in QFileDialog. These methods create QDialog objects, and calls exec(). This creates a new event loop, and this allows the same dialog to be opened multiple times without waiting for the old one to exit when the same shortcut is invoked.
  • In the future, I think that these static methods should not be used, because they are not compatible with QWindow. They either need to be reimplemented in a way that allows to set their transient parent window (being done in at least some of the current native dialogs), or maybe Qt labs platform dialogs should be used.