Add effect for global menu/title

[Erlan Sergaziev]

I like how Unity's combined global menu/title bar saves vertical space, however, currently there's no animation and thus it looks quite untidy.

I would suggest that when cursor enters the top bar, the title should slide upwards (like curtains) giving way for the menu.
Accordingly, when the mouse leaves the top bar, the title should slide down.

For the timing, I'd suggest that the upwards movement should be as fast as it can be noticed, but the sliding down can be somewhat slower.

[Mark Shuttleworth (sabdfl)]

 status confirmed

Please change the bug title from "slide effect" to just "effect". We
don't yet know exactly what effect we want, and so the bug would be
better as a placeholder for both the design work and then the
implementation.

Mark