Fixed view panning from [space] shortcut.
Crop reference (boundaries rectangle) + two grids + ui buttons

1. If you zoom out, you will see the crop reference. May be useful for
saving or exporting images.
2. Added 2 toggle buttons to select the 2 types of grid ( near
onion-skin, at the bottom right of the screen )
A new complete grid can be added in the future.
Merge pull request #201 from eVoLuTiVeMEDIA/master

Added grid references, some fixes. Code simplification and comments