Add copy button to the serial monitor

[Pieter12345]

Add a copy button to the serial monitor that copies the text in the serial monitor output area to the system clipboard. This is useful since ctrl-a + ctrl-c does not work when the serial monitor is spammed at a high rate.

Related: arduino/arduino-ide#1729.

[facchinm]

@ArduinoBot build this please

[ArduinoBot]

✅ Build completed.

Please test this code using one of the following:

⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-8640-BUILD-835-linux32.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-8640-BUILD-835-linux64.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-8640-BUILD-835-windows.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-8640-BUILD-835-macosx.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-8640-BUILD-835-linuxarm.tar.xz

ℹ️ The linuxarm build is still experimental and may not be always available.

[Pieter12345]

Rebased onto master (force-push). No additional changes were made.

[facchinm]

I just started to wonder if we can avoid the button by adding a contextual menu on the serial monitor itself with 2 entries:

• copy selection
• copy all
  Would it make sense? @per1234 @tigoe @mbanzi

[per1234]

I like the context menu idea. It seems like a fairly intuitive UI.

[tigoe]

My usual solution to this is to turn off autoscroll, then use a crtl-A, crtl-C. It makes me think it's worth adding a keyboard control for autoscroll, so you could use keys to do this all the way through. Would be better for accessibility too. context menus can be a pain for screen readers.

[Pieter12345]

Rebased this PR onto master.

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