Add unit guarding
adds artillery_explosion

Technically unnecessary, but using self_destruct on regular weapons in weapons.yaml might be a little confusing for modders. artillery_explosion is a more straight-forward label for art-exp1.
sound and explosion changes

Explosion sounds have been noticeably absent from the RA mod, artillery weapons only used the weak kaboom12, most other weapons had no ImpactSound at all, making even larger battles sound 'empty'.

Now, explosive artillery weapons (155m, 8Inch, SubMissile) use art-exp1 as animation and kaboom22 as impact sound. Furthermore, most cannons and rockets now use kaboom12 as ImpactSound. In one or two cases kaboom15 is used instead. Furthermore, for consistency Trail: smokey was disabled on Dragon and ChronoTusk.
Merge pull request #3281 from Mailaender/sdl-openal-mirrors

Added OpenAL and SDL Windows dependency DLLs to mirrors
Merge pull request #3192 from Mailaender/asset-browser

Created an Asset Browser to view and convert Sprites
Merge pull request #3233 from ScottNZ/follow

Add unit following/guarding
Merge pull request #3295 from reaperrr/patch-3

adding ImpactSounds to many weapons + other changes
Added ProximityCapture support for TD

closes #2951
made C&C hospital proximity capturable

fixes #3290