First bunch of merge from NetRadiantCustom

This is the port of my old garux-merge branch on current master. What is merged is still from 2017 and did not reached yet the point I wanted to reach before doing the first merge in master. The problem is that while I lack a lot of time to do this work, I started this work in 2019, already 3 days ago.

So, along the problem of merging a code from 2017 based on an old 2014 branch mixing things from this tree and new things in jumbo commits, the result of that merge on a master branch from 2019 now needs to be ported to current master. For example I had to port on changes that happened since I started the merge effort, like 80b20450 and other things like that.

So there is three levels of porting, old commits from 2017 based on branch from 2014 being ported to a branch from 2019 that is then ported to current master branch in 2022.

The original porting code was not easy as so much change happened upstream between 2014 and 2017 each commit has to be ported one by one sometime almost entirely rewriting them. And now, most of the commits from the old merge branch have to be ported one by one…

So, this is the current state. It's still merging very old branch of NetRadiant Custom but on latest upstream NetRadiant.

I verified it builds on Linux and that NetRadiant runs on Linux (I had to merge, port, build and test on every “already merged“ commit…), I'll try building and running on Windows and macOS later.

I would appreciate some testing.

Note that along some other things it merges the WASD camera key bind. You may have to delete existing shortcuts.ini from ~/.config/netradiant/ and ~/.config/netradiant/*/ to avoid some discrepancies.