Add single argument constructor to MessageDisplay. Refs #6202

Allows it to be used in the designer plugins more easily.
Move MessageDisplay to MantidWidgets where it belongs. Refs #6202
Add MessageDisplay to designer plugins. Refs #6202
Allow QtChannel to filter on a given source. Refs #6202
Replace log handling in SANS window with MessageDisplays. Refs #6202
Remove Poco logging from UserSubWindow. This is now all controlled

using the MessageDisplay & loggers. Refs #6202
Remove AbstractMantidLog & MantidLog classes. Refs #6206

They have been superceded by the QtSignal class.
Remove SignalChannel that is now superfluous. Refs #6202
Correct some spelling errors. Refs #6202