Serial Port dropdown - simpler handling multiple Arduinos / serial ports

I have a suggestion to improove work with several Arduinos: It is very cumbersome to change the serial port via the menu. A small dropdown list with the available serial ports next to the Serial Monitor button would be a great relief.

No files or tests are named. Start by inspecting the Arduino IDE 1.x serial-port menu and the Serial Monitor button, then trace how available ports and the active port are handled. Done means a nearby dropdown exposes available serial ports and selecting one changes the active port for multiple Arduinos.

Written by the indexing model from the issue text.