Simplify icon handling

Eliminate the pointless runtime command name -> icon map and just make
commands responsible for their icons. Don't bother predecoding and
caching icons since it wastes some RAM (~10 MB) and decoding icons takes
a trivial amount of time.

Fix a few places where icons were being set for the wrong command names.
Simplify HelpButton a little

Switch to flat_map for trivially less memory usage and just use a global
map rather than a heap-allocated map since it's not
(de)initialization-order sensitive.
Add 48x48 and 64x64 icons
Use retina icons for the main toolbar if first opened on a retina display
Dynamically switch to the retina toolbar icons as appropriate
Retinafy the video display
Update Indonesian translation
Replace the standard Lua include path entirely on Windows

The default Lua include path is encoded with the local charset rather
than UTF-8. This causes problems when the working directory includes
non-latin-1 characters which are valid in the local charset, as Aegisub
expects the string to be UTF-8. As the default path isn't very useful on
Windows (on other platforms it could potentially include things like
luarocks-installed modules), just clear it.

Closes #1676.
Fix potential use-after-frees on redo

Don't delete the old copy of the file until after the new version is
committed to avoid a use-after-free when code tries to do things with
the active line in a COMMIT_NEW handler, as just trying to avoid doing
that hasn't really worked out.
Merge remote-tracking branch 'upstream/master'