Remove gaps on confirmation button to fit 600px height

[facchinm]

Remove some unused space from Preference panel to be able to click OK and Cancel on 800x600px monitors.
On some configurations the OS could decorate the window and make it bigger than 600px; in this case, we should split the preferences into two tabs (argh) or make the window scrollable (even worse). Let's try this approach and see if it works.

Should solve #4753

[ArduinoBot]

✅ Build completed.

Please test this code using one of the following:

⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-6459-BUILD-705-linux32.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-6459-BUILD-705-linux64.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-6459-BUILD-705-windows.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-6459-BUILD-705-macosx.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-6459-BUILD-705-linuxarm.tar.xz

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

[rin67630]

Why did cmaglie delete the facchinm:smaller_preference_panel branch. We DO need it!

On IDE 1.8.7 it is still unusable with 600 Px display heigth.

[PaulStoffregen]

This fix was merged more than 1 year ago.

If there is still a problem with 1.8.7 or the latest nightly build, maybe open a new issue. Wouldn't hurt to reference this one and mention the fix wasn't enough for some displays. A screenshot of the actual problem and specific info about which operating system and desktop environment would also go a long way towards persuading anyone that this deserves more attention.

[cjdg]

Happening again in 1.8.9