Support image selection

Added a SelectType into OpMoveCursor to indicate whether it is a image
selection so we can draw our own selector after the OpMoveCursor is
excuted.

Thought about just draw the selector in the mouseup event, but it has
problem with undo a image deletion. the undo manager wouldn't know if the image
was selected before deletion.

known issue:
the selection box is moving up and down when using browser's zooming
functionality (working fine with the slider zoom though)