Rewrite code for toggle and mode actions, once again

We should run the callback not only when user changes the action
state via the menu, but also when it is changed in some other way
(i.e. programmatically using g_action_change_state).
Translate action name to lowercase in get/set callbacks

Also, avoid unneeded types casting by using GAction methods instead of
GSimpleAction ones.
Merge pull request #45 from mitya57/master

More fixes for the applet