Allow scrolling of the tab bar

Hi,

I'm using the Arduino IDE for a very large project (31 seperate Sketch files).
Switching through these tabs with the context menu on the right is working but it's not possible to scroll right inside the tab control as it is possible in other tab controls.

Because the scrolling is not implemented and I had to work with 31 tabs, I used my three monitors and resized the window to a width of 3600 pixels and noticed that the header has a maximum width of 3000 pixels and is then leaving ugly padding spaces on the left and right sides. (see screenshot)


Thanks for your fantastic work on Arduino,
Julian

The issue names no source files or tests. Start by locating the Arduino IDE tab-bar and header UI implementation, then reproduce the behavior with many Sketch tabs and a very wide window; done means the tab bar scrolls horizontally and the header no longer leaves the reported padding spaces.

Written by the indexing model from the issue text.