MainFrm: Get the source filter from the graph immediately after opening the main file.

Changed all code that depends on the source filter to take advantage of it.
Fixes issues with the wrong source filter being used when multiple source filters are on the graph.
CONTRIBUTING.md: fix forking link.
player.html: remove width completely.

It's handled in JS.
Initialize variables in the constructor.
Enable per-monitor DPI-aware on Windows 8.1.

Fixes #3356.
Add a Doxyfile to generate source doc.

WIP
Use DEBUG_NEW instead of new.
temp