.uno:CharBackgroundExt is supplementary to .uno:BackColor.

When .uno:BackColor is set, Writer turns into a 'watercan' mode, where the
user directly marks parts of the text with the wanted background color.

.uno:CharBackgroundExt then controls this watercan mode - dispatching it
toggles the watercan mode on/off, and also the StateChanged events reflect the
on/off mode accordingly.

Change-Id: I6472eb39129d1b1517fba14bad584cbd125e826a