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.
Use more generic interface

This allows to feed the method with other devices too.
Use QSaveFile if it is available

QSaveFile allows a safer way to write / overwrite files.

Tests with full file systems worked fine. Without QSaveFile it
horribly fails to save the current map leaving you with broken
files.

Fixes: #442.
Merge pull request #713 from Ablu/bugfix_442

Use QSaveFile when available for more reliable file saving
Merge pull request #711 from Ablu/add_tileset_button_in_dock

Provide a button to add tilesets in the Tilesets dock
Allow to add new animation frames by double click

Fixes: #706.