CMS-3911 New Grid - Re-selecting elements must not "reload" thumbnials

Updated an ImgEl cache breaker.
Timestamp can now be updated directly.
Added refresh method to `NamesAndIconView` class to update image.
Implement a simple handler for element update.
CMS-3911 New Grid - Re-selecting elements must not "reload" thumbnials

Optimized `ImgEl.setSrc()` method.
Renamed `TreeNode.hash` to `TreeNode.timestamp`.
Merge pull request #872 from enonic/CMS-3911

CMS-3911 New Grid - Re-selecting elements must not "reload" thumbnials