lib: Replace P_() by _()

As they're defined to the same thing anyway.
lib: Clean up gettext handling

Use glib's i18n support rather than copy pasting it. Note that
gdk_pixbuf_gettext() is kept as it's an exported symbol.
loaders: Simplify and shorten the loader names

https://bugzilla.gnome.org/show_bug.cgi?id=641106