Change LuaJIT's deployment target back to 10.4

Targetting 10.7 makes the error handler segfault, and targetting 10.4
appears to work fine.
Update ffms2
Write a BOM at the beginning of subtitles to make unpatched VSFilter happy

Closes #1795.
Don't version the executable on Linux

Release branches will continue to use versioned file names, but that
doesn't really make much sense for builds from master.
Use wx events for invoking on the main thread on OS X

Dispatching to the main queue with GCD does not work when there is a
modal dialog active.
Remove the automation thread priority stuff

It hasn't actually been used for anything for a while, and if a single
normal-priority thread can make your system unresponsive it's time to
upgrade to an OS that's isn't garbage.
Ensure the lua state gets closed when running tests

This is required for the gc metamethods to actually get run.