Build with -dead_strip on OS X

Bumps 3.5 MB off the binary and doesn't seem to break anything.
Add Bulgarian translation. Closes #1706.
Set extendRow even when active_line doesn't change

After shift-clicking on the grid the row to extend from doesn't equal
the active line, and clicking on the current active line to deselect all
other lines need to update the extend row.
Make deleterange not do totally incorrect things
Add resource strings for file associations

These weren't merged from 2.1.9 with the rest of the file association
code, which results in the installer trying to reference nonexistent
resources and things breaking.

Updates #1699.
Set ApplicationCompany for the Aegisub executable

Despite being completely unmentioned in the documentation this appears
to be required to show up in the default programs UI on Windows 8.1.

Closes #1699.