Graphics App: adding a setting to hide or display the app in the tablet/toolbar

[AleziaKurdis]

This make the tablet app to be launched or not based on new setting that control its present in the tablet/toolbar. It also make it able to discard or restore itself on the tablet/toolbar if the option is checked in the Setting menu. (See PR 1274 for the new setting)

By default the app will be visible.

TESTING:
This app must be tested once the PR 1274 will be present in the code.
(It hasn't been tested yet)

• Test if the app is visible and functional if the setting is find to true at startup.
• Test if the app is not visible if the setting is find to false at startup.
• Test if the app get visible and functional if the setting is changed to true at runtime.
• Test if the app get not visible if the setting is changed to false at runtime.

[ksuprynowicz]

I just tested it, works perfectly :)