Use GtkFileChooserDialog only when we need previews, default to
NativeFileChooser otherwise.
GtkFileChooserDialogs have a long list of issues, so lets only use it
if we gain something from it.
Flatpak should only run NativeFileChoosers because its sandboxed and
this is needed for security purposes. As a result of that, Flatpak Users
dont have image previews in the FileOpenDialogs
Refactor all FileChoosers for a more simple approach when we use them
Add a new SendFileDialog, so we dont have to put widgets into the FileChooser
which forces non-native Dialogs.
Philipp Hörist