Turn Gtk3 vclplugin #warnings into //FIXMEs

...to be able to remove the sledgehammer gb_Library_set_warnigns_not_errors.

Change-Id: Ib2cde5bbeaa1fb4dba629c51c493b77523455ff9
Disable only -Wdeprecated-declarations in Gtk3 vclplugin

Change-Id: I32558229af857ab705b872e9f1be2d84ca04204a