Migration from GTK 2.x to GTK 3

[fillods]

A porting guide is provided for the process of migrating grig from GTK2 to GTK 3:
https://docs.gtk.org/gtk3/migrating-2to3.html

Please announce beforehand your intent to tackle this task in order to not duplicate efforts and better organize ourselves.

[mattmelling]

Spent some time playing with porting grig gtk3 this evening to get a measure of what needs to be done. The main areas are:

• Refactoring drawing routines to use cairo_*; The gdk_gc_* functions were removed in GTK3, so all of the S-Meter and LCD drawing needs to be redone. I've had some success porting the S-Meter as a test, main blocker for me has been learning Cairo as I go.
• gtk_quit_add was removed which mainly seems to affect cleanup when widgets are destroyed, fixed for S-Meter which should work for other widgets
• Port combo boxes to new API, mostly seem to work but the functions to create/append items seems to have changed
• Some general gtk plumbing bits, for example a lot of places where we use gtk_box_pack_start seems to no longer compile
• A lot of compiler warnings, mostly deprecations
• A ton of runtime warnings from GTK, I had to comment out a lot of stuff to get it to build so there are a lot of null pointer warnings where things are a bit broken, likely these will come down as we get stuff working.

I believe it is possible to make these changes in a way that would support both gtk 2 and 3 (certainly the Cairo stuff), so we don't run the risk of changing everything all at once and breaking things.

Having made a start with the S-Meter, I'm happy to tackle the other bits. I'm not particularly experienced with GTK so may require guidance on any particularly rough parts.

[mattmelling]

Whoops, sorry for the commit spam, was trying to organise my local branches

The S-Meter has been converted to cairo, and I am most of the way done with the LCD. Just need to finalise the manual entry parts. I hope to have a PR ready over the weekend.

[luzpaz]

Any chance there can be a gtk4 port ?