#873 Display the current (implicitly detected) master document

[Tim Hoffmann]

Resulting from a discussion on [bugs:#1333]

It would be nice, if the current document to be compiled would be visible in the GUI.

Note: this needs some restructuring of the code. Currently, the document to be compiled is just determined before running the compiler (pull-principle). If you want to have an up-to-date display, you have to update it from every event that possible might change the master.