gproject: Lots of new features and improvements

* Added the "external directory" feature
* Added find tag by name
* Added ignored file patterns
* Performance improvements in tag generation using updated Geany API
* Automatic tag generation for projects with less than 500 files (configurable)
* Follow active editor on by default
* Treat empty "file patterns" in project properties as * (everything)
* New toolbar icons
* Fixed missing icons under Windows (thanks to Enrico)
* Lots of minor iprovements, bugfixes (and probably new bug introductions)