Improve keyboard navigation with KP_BEGIN and KP_END

[Hartmnt]

Description

In #5984 the accessibility of the entire application was overhauled. Mumble was made keyboard navigable. However, I failed to notice that KP_BEGIN and KP_END were also used by screen reader users for keyboard navigation.

Thanks to user squeaktoy for bringing this to my attention: #5984 (comment)

We need to go through all UI elements and see how they react to pressing KP_BEGIN and KP_END (or "Home" and "End")

Mumble version

1.5.634

Mumble component

Client

Reproducible?

Yes

[squeaktoy]

I did notice that KP_BEGIN works differently on Windows/NVDA than it does on Linux/Orca. On Orca it reads out useless info while on NVDA it does read what you would get if you focus the element through regular navigation.