GUI: Tab scroll buttons should be disabled when they have no effect
|Reported by:||salty-horse||Owned by:||sev-|
When the tab widget has too many tabs than can be displayed on screen, scroll buttons ("") 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.
Ticket imported from: bugs/7123.