BUG: Add error log button to mainwindow

Each time a Warning/Error/Fatal message is logged, the tool button
associated with the error icon is highlighted.

If the user either

 (1) click on the button

or

 (2) focus on the error log widget in case it was already visible, the
 icon associated with the tool button is "un-highlighted".

Fixes #1349
Fixes #2537

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22304 3bd1e089-480b-0410-8dfb-8563597acbee