Enabling Frescobaldi to load external extensions

[Urs Liska]

I have (finally) started to work towards making it possible to extend 
Frescobaldi with extensions. I have opened a Pull Request at 
https://github.com/wbsoft/frescobaldi/pull/1129 and invite anyone 
interested in looking at and discussing it. Probably this is mostly 
accessible for people who can look at the Python, but there may also be 
some usability aspects that would benefit from general, user-perspective 
discussion. A heavily commented sample extension can be found at 
https://github.com/frescobaldi-extensions/sample-extension, and at some 
point there will be a tutorial on that repository's Wiki pages.

If my basic approach is accepted by the other developers this will allow 
the creation of very simple Python packages that can be loaded as 
extensions in Frescobaldi. They will have access to the whole 
functionality that is available within Frescobaldi, but of course using 
this will require the knowledge, so it won't be trivial for everybody to 
create their own extensions (but it'll be a simpler task than it is now 
to find *someone* to create extensions, e.g. as a commission). What I've 
found so far (which was one of the motivations to actually do something) 
is that Frescobaldi is a perfect environment to place some project 
management GUIs into, which the extension API will make much simpler.

Urs