stb_textedit: Add support for alternative keyboard shortcuts for text/line start/end.

Adds support for having multiple keyboard shortcuts for the text/line start/end commands.

On OS X, there's multiple standard shortcuts for this:

TEXTSTART is "Home" or "Cmd+<Up>"
TEXTEND is "End" or "Cmd+<Down>"
LINESTART is "Ctrl-A" or "Cmd+<Left>"
LINEEND is "Ctrl-E" or "Cmd+<Right>"

This adds TEXTSTART2 etc.; when defined, these set the secondary keyboard shortcut for the given command.