Remove custom encoding file dialog in favour of native file dialogs

See discussion for what the final MR does

This PR adds an option in the preferences to make meld use the system file dialogs where ever possible. This unifies the look of the file choosers for file comparisons and directory comparisons in non GTK environments at the expense on the file encoding setting in the custom file chooser.

Technical stuff:
This is done by adding three more GtkFileChooserButtons which use the systems/native file chooser dialogs. This has to be done that way since the dialog property of the GtkFileChooserButton can not be set/changed after its creation. Changing the option in the preferences simply changes which set of buttons is displayed.