On OS X the Command key is used for common keybindings instead
of Ctrl. Introduce a new macro, GEANY_PRIMARY_MOD_MASK that
represents the Command key on OS X and Ctrl on other platforms.

For some events, such as mouse key press, GDK_MOD2_MASK is returned
for the Command key by GTK instead of GDK_META_MASK (which is returned
when Command is pressed together with some other key). To hide this
behavior from users, introduce keybindings_get_modifiers() which can be
used instead of gtk_accelerator_get_default_mod_mask() and which
inserts GDK_META_MASK when GDK_MOD2_MASK is found in the mask
on OS X.