Opening the Serial Monitor while uploading is not a good idea

[NicoHood]

This should be blocked or it should be opened grayed out and then starts to listen.

[matthijskooijman]

This is related to #3771, a rewrite to fix that one should probably also pick up this issue.

[facchinm]

Solved by #4482 (commit 3f07b8a)

[facchinm]

Fixed by merging 6d5597b