Merge branch 'lua-more-stuff' into 'master'

Lua more stuff

More new Lua features and fixes:
* Most Lua functions that deal with stuff that exists only in levels now should spout Lua errors instead of crashing the game if you try to use them outside of levels. Likewise, accessing any of the tables that contain level-only stuff (players, sectors, lines, etc etc) spouts Lua errors too outside of levels.
* `userdataType(variable)` now exists: this function simply returns the type of the userdata variable given as a string (e.g. `userdataType(players[0])` returns "player_t", `userdataType(sectors[0])` returns "sector_t"). This also includes "minor" userdata types for array members of other userdata types, such as .powers of player_t variables or .lines of sector_t variables (which would give the strings "player_t.powers" and "sector_t.lines" respectively).
* The Lua hook "MobjMoveBlocked" now exists: functions for this hook are called whenever a mobj attempts to move horizontally but is blocked by a wall or solid mobj (or whatever else can cause P_TryMove to return false, assuming it doesn't remove the mobj). In theory this hook could be very useful for behaviour such as sliding or bouncing off walls without the need of flags like MF_SLIDEME or MF_BOUNCE etc. Format for use is just like most generic mobj hooks: `addHook("MobjMoveBlocked", functionname, MT_OBJECTTYPE)`, where `functionname` is a function that takes a single mobj_t argument.

See MonsterIestyn/lua-more-stuff on the FTP for a test exe (srb2win-lua-more-stuff.exe) and some test scripts for the above changes.

See merge request !67