Project dependencies: Search for files relative to the master document

In LaTeX, \input and \include arguments are relative to the master document, not relative to the file in which they are included. However, currently dependencies are searched for relative to the file that contains the \input argument. This leads to displaying issues in the project tree as described in Bug 297675.

This MR checks whether a master document exists, and if yes, searches for the dependencies relative to the master document. Also, whenever a new master document is set (or unset), the project tree is updated.

In my opinion, this is not optimal yet, as the master document has to be set explicitly, but this seems to be a minor inconvenience.