Fix transparency issues with GTK3 plugins

This patch uses some GTK3 CSS magic written by Simon Steinbeiß to
make the GtkPlug button in the panel transparent. Small changes
were made to the wrapper_plug_draw code as well, i.e. no need to
check GTK_WIDGET_IS_DRAWABLE since gtk does this before calling
the draw signal. It also transforms the draw coordinates from
widget-relative back to window-relative.