Fix a couple of selection issues

* In FireFox if an image has no text around it, click on either side of the image resulting the same selection get returned. focusNode: image, focusOffset: 0. To workaround it, move the cursor to the next walkable position when clicking on the right side of an image.

* Another corner case is that when click on the right side of an image that has no text elements, non-FireFox browsers will retrn focusNode: frame, focusOffset: 1 as the selection. Since this is not a valid cursor position, move the cursor to the next walkable position after the frame node.
Merge pull request #294 from kevinlli/selectionFix

Fix a couple of selection issues