Make 3D mode actually work again.

It appears it was broken (at least on Linux) because it "captures" the
mouse, by moving it to the center of the window every frame -- but this
movement caused a new mouse movement event to fire, so the event queue
was constantly full, and paint events never got through.

There was even code to fix this problem, but it only existed on an SFML
1.x compat codepath, and was deleted a few months ago.  Apparently it's
needed with SFML 2.0, too, at least with GTK.  Whoops.
Merge pull request #209 from eevee/fix-3d-mode

Make 3D mode actually work again.
Only enable Boom extensions in Doom map format

Specials above 255 are not supported in Hexen or in UDMF-ZDoom namespace. (And the config does not cover ZDoomTranslated or vanilla namespaces.)
Fix some bad format specifiers.
Always show shortcuts in menus, if available.
Add a tooltip to the floor/ceiling height textboxes, and fix Tab/Ctrl-A/etc.
Clarify the wording of the auto-open behavior -- I thought it was referring to /.
Merge pull request #211 from eevee/minor-ui-fixes

Minor UI fixes
ZDoom language configurations update
Merge pull request #213 from Blue-Shadow/master

ZDoom language configurations update
Put linebreak after textureceiling property in UDMF

GZDB cannot handle maps with more than one property per line.
Tighter checks on TGA and Beta graphics

Fixes #214.
Merge branch 'master' of https://github.com/sirjuddington/SLADE into osx