Feature Request: Accept alternative .hex runtime

[PropGit]

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).

[yanghao]

This makes a lot of things easier, for example, currently the default v2 in python.microbit.org (version 2.1.1) does not yet fix the time_pulse_us() problem, which would be highly beneficial if a different runtime can be selected. (see: microbit-foundation/micropython-microbit-v2#26).