Add an erase button into GridPaint activity

[llaske]

It could be nice to add an erase button in GridPaint activity toolbar.
This button should be visible only on the paint screen (not on the global view with all paintings).
When you click on it, it will paint all triangles of the current painting in white.
The appearance of the button must be the same than the Erase button in Paint or Physics activity.

Code of GridPaint activity is here: https://github.com/llaske/sugarizer/tree/dev/activities/Gridpaint.activity

[llaske]

Fixed in #249