Added context menu for Eraser
Allow small/medium/large size
Hold the eraser button to open the context menu
Note: in current stage there exist an usability defect. When you click
the pencil button and then hold the eraser button, the context menu for
PENCIL will come out. You need to click the button first, and then click
and hold the same button again in order to open the menu. Currently
there is no solution for this...