Feature Request: Accept alternative .hex runtime

Request that the Load/Save dialog accept custom .hex runtime (MicroPython interpreter runtime) that it uses for that project in-place of it's built-in runtime. This would be similar to functionality already in the Mu editor and would allow users to enjoy the same benefits.

This thought may be similar to subpostings in Issue #26

Suggestions:
- the runtime appears in the Project files list (file system list) of the Load/Save dialog.
- limit to 1 runtime per project (of course); new runtimes replace old runtime
- .hex file loading properly parses out user .py, modules, and runtime, if provided .hex is an entire project (rather than just a MicroPython runtime).