Support erasing using drag

[hari]

Currently, drag supports filling only. We need to implement erasing as well. We can do this using one of the following methods:

1. Use the action on the first pixel i.e. if the first pixel was filled the dragging fills and vice-versa.
2. Alternatively, we can use the dedicated mode option to fill or erase.

Or we should have Undo or Redo button.