Made variable name easier to understand
Provide a shortcut to add tilesets from the dock

This only works for non external tilesets.
External ones would require an icon. But it should also be
considered wether another button in the dock would not be too much.

Fixes: #695.
Merge pull request #711 from Ablu/add_tileset_button_in_dock

Provide a button to add tilesets in the Tilesets dock