ProHelper TODOs

[CRImier]

I've developed ProHelper - a helper for running long processes when working on the Avrdude app. It allows you to run an external process of some sort in background, write to its stdio and read its stdout, track its progress and terminate it when needed. However, we need to improve it so that other developers can use it more conveniently:

• Needs to implement terminal-less running (for now, it's just raising NotImplementedErrors with use_terminal=False because I didn't implement that part and only needed running stuff with a terminal)
• Needs documentation for developers, usage examples in "How to"
• Needs to have stdout reading and writing properly covered with tests
• Needs to have shell=True covered with tests
• Consider renaming it - maybe to ProcessHelper?
• Needs a way to pass signals (i.e. USR1) to it
• Needs a way to get PID of the process