Update map via QT signal instead of in ros thread

Resolved issues when running RViz in rqt where the incomingMap callback
is not issued from RViz's main QThread causing a crash in Ogre. Map
updates are now handled by emitting a signal to update the map from the
callback thread.
Merge pull request #819 from ros-visualization/pr_817_indigo

[fp to indigo] Update map via QT signal instead of in ros thread