Add a 'highlighted' pseudo-state to QTabBar tabs

A fairly common scenario for QTabBar tabs is to make them blink in response to something happening in a tab. Right now options to change individual tab's look are limited - it's easy to make a tab icon blink, but changing a single tab's background color is difficult (usually involving subclassing widgets and overriding paint events), and QTabBar methods to e.g. change tab's text seem to be overridden by any CSS affecting the tab.

Giving the tab sub-control a 'highlighted' pseudo-state (or possibly a property, if making it a state could cause issues) would make it much easier to blink a tab, while putting responsibility for actual look of the blinking in the CSS.