Hot corner ehancement requests

[CaverBruce]

Nice feature, following on from #94 ... Next steps to consider for hot corners;

1. Allow the user to customize the size of the hot corner. Using on a small device with curved edges, the edges of the screen are perversely insensitive, so anything at the edge is hard to activate. Also, all of the capacitative styli i have found are pretty blunt instruments. Both are reasons to increase the size of the hot corners on the particular device i am using.
2. Turn off the hot corner visibility when it is not relevant. In 'pan mode' (second button from left) the hot corners are redundant, as touching anywhere on the canvas will pan, therefore the corners should not be visible (and should not activate). That they do activate can cause the user confusion as to the mode they are in. Conversely removing them when they are redundant gives the user additional reinforcement that they are in the 'pan mode'.
3. Consider moving the lower two hot corners several pixels up away from the button bar. 70% of the time I activate the adjacent button (black colour or zoom +) instead of the hot corner move mode.
4. Prevent activation of hot corner if a line is currently being drawn. If in draw mode and I am drawing a line and the stylus passes through one of the hot corner zones, the line drawing is terminated and the canvas starts panning instead. I would prefer that the canvas NOT pan in this scenario, and continue drawing the line instead.