Open dialog filter registration, other tweaks

File selection from Open dialog is now plugin-aware and functions the
same way as opening from the project tree, meaning the File->Open
submenu is no longer needed and has been collapsed to a single "Open"
command.  Import now uses the same filter list as Open.  Removed
wildcard filter from script editor and added additional text types to
compensate - too easy to corrupt binary files if the script editor opens
them.  I think we need a hex editor plugin...
Merge pull request #9 from powercommand/master

Open Dialog filter registration