webadmin: resizable dialogs - missing resize symbol

On resizable dialogs [1], the resize symbol [2]
(on bottom-right corner) was missing.

Hence:
* Extracted the image into 'branding/../images/dialog' folder.
* Added '.gwt-DialogBox .dialogBottomRight.resize' style
  to 'gwt_common.css' file.
* Modified ResizableDialogBox file accordingly.

[1] only Disk and Storage dialogs are currently resizable
[2] screenshot - http://i.imgur.com/nnMwcPh.png

Change-Id: I7e9cc8a3140ca62c677b9ac53c00c5460d3b0009
Signed-off-by: Daniel Erez <derez@redhat.com>