heretic: Fix plat_t read when loading savegames.

The fix-up of the thinker function was (unnecessarily) guarded by an
if() condition that meant it was not being reset properly on load.
This caused moving platforms to be not restored properly when loading
savegames.

Fixes #343. Thanks to romeroyakovlev for the bug report.
doom: Add -dehlump parameter to load DEHACKED lumps.

Lots of otherwise Vanilla-compatible WADs contain DEHACKED lumps.
Allow these to be loaded by adding a -dehlump command line parameter.
Thanks to Fabian Greffrath for the suggestion (fixes #349).
heretic/hexen: Fix pause key remapping.

Heretic and Hexen had the pause key hard-coded to KEY_PAUSE when they
should actually use key_pause so that they can be remapped by the
user. Thanks to Fabian Greffrath for the bug report (fixes #337).
doom: Fix behavior of M_EPI4 menu item.

Ultimate Doom's doom.exe will crash when run with an IWAD that doesn't
contain M_EPI4 (like the stock v1.9 IWAD).
Thanks to Alexandre Xavier for the bug report (fixes #344).
Update URLs to Chocolate Doom Github project.
hexen: Increase maximum level number to MAP60.

Hexen: Deathkings of the Dark Citadel uses levels up to MAP60, not
MAP40 as used in hexen.wad. Bump up the limit for Hexen to MAP60 so
that it is possible to play multiplayer games with hexdd.wad loaded.

Thanks to Rhiyo for the bug report. This fixes #312.
video: Center mouse when grabbing cursor.

When releasing mouse grab (activating menu etc.) we move the mouse
cursor to a non-distracting location at the bottom-right of the
screen. But this was causing abrupt jerks in mouse movement when
re-grabbing the mouse again. Center the mouse when turning on grab so
that this doesn't happen.

This fixes #283.
readme: Fix note about CMDLINE to be correct.

CMDLINE is now found in a subdirectory in source releases, making
the note in README inaccurate. Reword this to be correct.

Thanks to spicyjack for reporting this bug. This fixes #323.