Fix file paths conflicts between gtk2 and gtk3 builds

[wandrien]

$ find libfm-gtk*/usr/lib 
libfm-gtk2/usr/lib
libfm-gtk2/usr/lib/libfm-gtk.so
libfm-gtk2/usr/lib/libfm
libfm-gtk2/usr/lib/libfm/modules
libfm-gtk2/usr/lib/libfm/modules/gtk-menu-trash.so
libfm-gtk2/usr/lib/libfm/modules/gtk-fileprop-x-desktop.so
libfm-gtk2/usr/lib/libfm/modules/gtk-fileprop-x-shortcut.so
libfm-gtk2/usr/lib/libfm/modules/gtk-menu-actions.so
libfm-gtk2/usr/lib/pkgconfig
libfm-gtk2/usr/lib/pkgconfig/libfm-gtk.pc
libfm-gtk2/usr/lib/libfm-gtk.so.4.1.3
libfm-gtk2/usr/lib/libfm-gtk.so.4
libfm-gtk3/usr/lib
libfm-gtk3/usr/lib/libfm
libfm-gtk3/usr/lib/libfm/modules
libfm-gtk3/usr/lib/libfm/modules/gtk-menu-trash.so
libfm-gtk3/usr/lib/libfm/modules/gtk-fileprop-x-desktop.so
libfm-gtk3/usr/lib/libfm/modules/gtk-fileprop-x-shortcut.so
libfm-gtk3/usr/lib/libfm/modules/gtk-menu-actions.so
libfm-gtk3/usr/lib/pkgconfig
libfm-gtk3/usr/lib/pkgconfig/libfm-gtk3.pc
libfm-gtk3/usr/lib/libfm-gtk3.so.4.1.3
libfm-gtk3/usr/lib/libfm-gtk3.so
libfm-gtk3/usr/lib/libfm-gtk3.so.4

1. lib/libfm/modules/ should probably be lib/libfm-gtk2/modules/ and lib/libfm-gtk3/modules/.
2. The modules installation path should include ABI version index the plugins built for. (lib/libfm-gtk2/modules-0/, lib/libfm-gtk3/modules-0/).

This change requires rebuilding any 3rd-party plugin modules. Luckily, we don't seem to have any at all.