MDI interface needs separate board/serial port control

[bhagman]

Sometimes it's nice to have two sketches open, one for each board connected, and be able to build and upload without having to select the board and serial port each time.

This is really low priority, as most people will not be working with two boards (or possibly more) at the same time.

This applies to the editor selection of board/serial port, as well as the Serial Monitor, as the Serial Monitor window works as only a single instance.