Add a scrolling bar to the tab menu

@ffissore This is a follow up of the issue https://github.com/arduino/Arduino/issues/2765 AKA "**How do you know which tab you're editing?**" that was resolved with https://github.com/arduino/Arduino/commit/2339049c0fd0cd7d215adc659f044a999a9dfeca, thank you again.

Working with the IDE is now (in my case) day and night. But there is a last one improvement I wish to see in the IDE: a scrolling bar in the tab menu. I think the top and bottom arrow in the tab menu should be replaced by a scroll bar. Not only it's more intuitive, it is **_WAY SPEEDIER**_.

Also, the tab bar should have a left and right arrow to effectively scroll the tabs (like asked in https://github.com/arduino/Arduino/issues/1193, it was in 2012).

Is this feasible?

Start by reviewing issue #2765 and commit 2339049c0fd0cd7d215adc659f044a999a9dfeca to understand the existing tab-menu behavior, then trace the Arduino IDE 1.x tab-menu entry point. The work is complete when the menu has a scrollbar and the tab bar supports left and right scrolling as requested here and in issue #1193.

Written by the indexing model from the issue text.