Force loading of plugins *after* evaluating the script

Instead of before, to make ClearAutoloadDirs etc. work.  The only case
where AvsP needs to call an external function before evaluating the
script is the vpy hack, and in that case ClearAutoloadDirs won't be
ever called, so the changes are not very ugly.
Don't accept dots in a file extension on 'Extension templates'
Fix opening of script files with BOM
Improve 04874ed5
Fix opening the crop editor when the preview clip is not ready

Bug introduced in d0cb9ab7
Stop video playback when starting 'run analysis pass'
Couple of fixes for highlighting of triple-quoted strings

- Quotes sometimes were intrepreted as delimiters for both ends of the
  string simultaneously, e.g """" was highlighted as if it were """"""
- AviSynth interprets """"""" as '"' etc.
Add support for 64-bit AviSynth+

- Add new bindings for AviSynth using cffi. This is needed to workaround a
  limitation in libffi that prevents using the ctypes bindings with 64-bit
  Python, see <http://bugs.python.org/issue16575>.
- Only interface 5 is supported. Therefore AvxSynth is not currently
  supported.
- As the new bindings require a compiler, only use them for x86-64.
- Update the ctypes bindings to use the same interface as the new ones
  (no need of dealing with AVS_Value directly anymore).
Show OS and architecture on 'about AvsPmod'
Update the build/packaging system for AviSynth+ x86-64
Attempt to locate editbin.exe