Context Navigation

SingleWindowLayout

-              v63
+              v64
 First of all, I fixed the size of the automatic notebook which is used for the startup layout. The related PR https://github.com/OSGeo/grass/pull/1775 has still some minor shortcomings with cmd warnings messages but we decided to let it be for now since it would require more intervention in the construction of the map panel status bar widget which is anyway likely to be very simplified or replaced by the settings dialog in the future.
 Secondly, I implemented the new changing tab event for the center AuiNotebook. You can have a look at PR https://github.com/OSGeo/grass/pull/1780 which is already merged. To provide the fully working GRASS Single-Window GUI prototype, I also opened the PR [https://github.com/OSGeo/grass/pull/1789] which addresses several smaller things related to the AuiNotebook closing event. This PR is not merged yet but I plan to have it done by the end of GSoC. Merging this PR will close the Issue https://github.com/OSGeo/grass/issues/1750 with the new AuiNotebook events.
 Thirdly, I moved the NewDisplay button to the upper Tools toolbar, see the PR https://github.com/OSGeo/grass/pull/1783 (merged).
 Lastly, each layer tree is about to have one independent toolbar. I have coded it in the PR https://github.com/OSGeo/grass/pull/1785 which is not in the master branch yet.