First of all, great tool!!

Wish: add a check on the 'slice by cell' settings before trying to slice 

Why:
Currently if cell size, margin and gap settings are wider and/or higher then the loaded sprite sheet,
an error popup warning is shown with label ' Out of memory'.
This isn't much user friendly and also doesn't give the user any information he/she might need to 'solve' the issue .

Reproduction:
Given I load a sprite sheet
When I select the option 'Slice by cell size' 
And I set either or both cell size for Width and or Height 1 pixel wider /higher
And optional I set a gap or margin
Then the canvas to be cut is wider and or higher then the loaded sprite sheet
When I click the Slice button
Then the program shows a ' Out of memory'  error popup

Expected situation:
After clicking the Slice button, but before starting the actual slice proces, run a check on the cell size, margin &
gap settings for both width and height to assert they are still within the dimensions of the loaded sprite sheet.
If check fails, show warning popup with info regarding wrong cell size settings.