add all.h and use in place of diablo.h

[mewmew]

Now diablo.h is treated in the same way as all other header files of
Source, as it only contains the declarations of global variables and
functions of diablo.cpp.

Besides consistency, this also enables mods to include diablo.h just
like any other header file without having to include every header file
(and without having to include C++ specific aspects of the now all.h).

This PR supersedes #448.

[mewmew]

Note: this change is useful for mod developers as it enables linking against DevilutionX directly from C, thus increasing interoperability. Further background on modability is given in #447 (comment)

[AJenbo]

Didn't we decide to flip this and diasurgical/devilution#1848 ?

I can try and get it done this weekend. Here are comparisons of build times on the various CI before and after the PR:

	Before	After	Diff
Switch	00:04:54	00:04:44	-00:00:10
Unit tests	00:02:26	00:02:28	00:00:02
Linux 32bit	00:02:19	00:02:10	-00:00:09
Amiga m68k	00:00:59	00:00:49	-00:00:10
Linux SDL1	00:01:40	00:02:24	00:00:44
Linux	00:01:40	00:02:11	00:00:31
Windows GCC	00:01:52	00:02:32	00:00:40
Mac 10.3	00:03:29	00:03:26	-00:00:03
VS 2017	00:03:33	00:02:53	-00:00:40

[AJenbo]

Gave it a stab, but #1848 will require us to first remove miniwin (just one file left)

[AJenbo]

Moved to upstream as it makes more sense there, I think the different build times are down to randomly landing on different build servers.