Svg edit - multiple improvements, and start of transferring edits to the source

First of all I apologize for this rather overwhelming pull request! The good part is that @uliska and myself have worked hard on this and made some progress; the bad part is that there are many commits, and worse that some of them are quite messy. @uliska has offered the option to clean up history if that is preferable. 

Except added functionality to the SVG viewer this also includes a new panel called the Object Editor which works as a kind of mediator between the source code and the SVG view. This should for now also be hidden for stable releases. What we have so far is some basic ability to view and insert an offset made by dragging elements in the SVG view. 

This is all very much still a work in progress. At the moment it works best for moving text elements because we started in that end. More info can be found in this collection of issues and discussions: https://github.com/PeterBjuhr/frescobaldi/issues?state=open