#7123 GUI: Tab scroll buttons should be disabled when they have no effect

[Ori Avtalion]

When the tab widget has too many tabs than can be displayed on screen, scroll buttons ("<" and ">") are added to it. For an example, click the "Options" button in the launcher, when using a low-enough resolution.

The scroll buttons scroll the visible tabs. When the tab "list" is on the right/left edge, the right/left button is clickable, but does nothing. It should be disabled instead of being clickable.