Add horizontal scrolling to open files tab line

This is somewhat related to the issue https://musescore.org/en/node/153011 to ease the navigation experience when multiple files are open.
As opposed to the other issue that suggests a dropdown menu listing all the open files, I would find it extremely useful, and likely easier to implement, to add a scrollbar (or scroll effect -- similar to the one in firefox) to the open tabs.

Thanks!