Frescobaldi feature testing (Document font dialog)

[Urs Liska]

Hi all,

I've finally managed to do some work on Frescobaldi again, picking up on a 
completely new "Document Fonts" dialog I had started some time ago. I think it 
is approaching a point where it can be merged, and therefore I'd be glad about 
people testing it who

* Use Frescobaldi from Git
* Use LilyPond >= 2.19.12
* Use alternative notation fonts

... or are interested/willing in doing so.

The dialog replaces the old "Set Document Fonts" dialog which only provided an 
interface to selecting text fonts through drop down boxes, which is extremely 
inconvenient with many fonts installed. It also replaces the barely usable 
"Show available fonts" tool that simply presented the output of LilyPond's 
compilation with -dshow-available-fonts.

The new dialog lists text fonts in a tree view that can be filtered with an 
incremental search field, plus there's a text sample for every font.
Notation fonts can be listed and (to some extent) managed by "installing" and 
"removing" them from a given LilyPond installation. An option to download new 
fonts will be added later.

The main feature of the dialog is to generate a font settings command to be 
inserted in the current document (at a later point this will be accessible from 
the Score Wizard too), and a preview to see the results of the selected fonts 
(either applied to a set of provided sample scores, the "current document" or 
arbitrarily opened custom files).

See the latest entry to https://github.com/frescobaldi/frescobaldi/pull/1075 
for a few screencasts, and 
https://github.com/frescobaldi/frescobaldi/issues/1164 for the list of 
completed and outstanding features and issues.

Best
Urs