Restore Defaults is applied only on OK pressed

[FirasH]

Fixes #1415

[FirasH]

Includes also a missing space in the Revert to saved confirm dialog.

[FirasH]

There is a functional issue with the current implementation.

Some values have their signal connected to a function which does change the settings of Synfig.
This means that if for example autosave is disabled, when Cancel is pressed it won't be reverted to current settings:

synfig/synfig-studio/src/gui/dialogs/dialog_setup.cpp

Lines 829 to 834 in 6d3f75f

	void
	Dialog_Setup::on_autobackup_changed()
	{
	auto_backup_interval.set_sensitive(toggle_autobackup.get_active());
	App::auto_recover->set_enabled(toggle_autobackup.get_active());
	}

Probably the best way to deal with it is to remove those parts that in practice don't give any advantage to the user.

[FirasH]

@ice0 this request is ready. Do you need me to force-push it?

[ice0]

@FirasH yes, can you rebase it?

[FirasH]

@FirasH yes, can you rebase it?

Done 👍

[ice0]

Merged. Thank you!