gedit: port GeditFileChooserDialogGtk to GtkFileChooserNative

The intent is to allow GEdit to make use of portals when running under confinement, removing the need to give it wide file system access.

The encodings and newline combo boxes are converted to use the newer "choice" API, which handles everything except adding new encodings to the list.

The file filters code also remains mostly unchanged. The use of a custom file filter will need some changes too.

This may also allow us to remove the OS X specific file chooser code, since GtkFileChooserNative will use native dialogs on that platform (and Windows too).