Switch file from C++ to Objective-C++
Handle non-mouse/key events while running commands

Fixes textmate/diff.tmbundle#5
Restore default signal handlers for commands

Before this commit it wasn’t possible to SIGINT / SIGTERM commands launched from TextMate since TextMate itself has a custom handler for these signals (so they are set to be ignored) — this included pressing ⌃C in the HTML output window (for commands using the TextMate.Executor API).
Allow aborting commands by pressing ⌘. or ⌃C