state: support querying whether virtual modifiers are active Previously it was not possible to query the status of virtual modifiers with the following functions: - `xkb_state_mod_index_is_active` - `xkb_state_mod_indices_are_active` - `xkb_state_mod_name_is_active` - `xkb_state_mod_names_are_active` Note that it may *overmatch* if some modifier mappings overlap. For example, the default “us” PC layout maps both “Alt” and “Meta” to the real modifier “Mod1”; thus “Mod1”, “Alt” and “Meta” modifiers will return the same result with these functions.