Allow Serial Monitor to be Viewed After Disconnecting

[Logicer16]

Is your feature request related to a problem? Please describe.
When removing a board that had its output displayed on the serial monitor, the serial monitor will clear and begin trying to repeatedly reconnect. For example, a user may want to analyse the output of an board without being bombarded with now messages, or where a board is a shared resource and serial output is used to get debug information that can be worked on while someone else uses the board.

Describe the solution you'd like
The serial monitor should not clear after the board is disconnected and instead. Also there should be an option to enable auto-reconnect to stop messages potentially blocking the serial monitor (similar to #314), and instead show a single message giving the user the option to begin the auto-reconnect process.

Describe alternatives you've considered

• Turning off auto-scroll allows the messages to be viewed but not highlighted or copied (Serial Monitor text selection starts from beginning instead of location selected by mouse cursor #247, Unable to select content from serial monitor #167).
• Also, as this could be done in the original Arduino IDE, a user could have both installed and use 2.x for development and 1.x for the serial monitor though that would be extremely inconvenient.

[per1234]

This was fixed somewhere between the 2.0.0-rc1 and 2.0.0-rc2 releases. I guess it must have been one of the changes in #661