BUG: #2932, #3001, #3002 Several improvements to the editor.

#2932 - new Pixel Mode in Paint tool allows precise control when zoomed
in close to the image.

Add a set of hotkeys per #3002 for editor-wide functions, also
new +/- control on the paint brush to adjust size while it is active.

Per #3002 if a master and merge volumes already have been selected,
don't reset them when entering the editor.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21874 3bd1e089-480b-0410-8dfb-8563597acbee