Support image selection

*Added a ImageSelector class to draw and clear image selection

Known issue:
The selection box is moving up and down when using browser's zooming functionality. but it is working fine with the slider zoom
Only respond to the following keyboard events for image selection.
esc, tab, delete, backspace, undo/redo and copy/cut.

Note: MS Word behave differently from LO/AOO when image is selected.
When press letters,numbers&symbols, MS Word inserts the text just before the
image; where LO/AOO ignores all of them.
Also you can extend selectiion using keyboard shortcuts (e.g
shift+left) in MS Word, but you cannot do so in LO/AOO.

TODO:
Esc - deselect image
Tab - move to next frame in the document
Disable font and paragraph formating toolbar items on the editor when image is selected.

LO/AOO actually hides those options.