Introduce new Extract actions

Now that we have introduced the new Archive and File menus, it makes sense to introduce
different Extract actions as well, one for each menu:

1. The Extract action in the Archive menu, which always extracts the whole
   archive (even if the user selected some file).
2. The Extract action in the File menu, which is only enabled if some file
   has been selected, and extracts only those files.

The Extract action in the toolbar is the very same Extract
action that Ark currently provides: it allows to extract the whole
archive, or only the selected files (if any).

Differential Revision: D795

GUI: