Rename refresh rate

[daxpedda]

While working on #3802 I wondered about VideoModeHandle::refresh_rate_millihertz()s really long name.
WDYT about renaming it to refresh_rate()?
Alternatively refresh_rate_mh()?

Requires #3802.

[kchibisov]

I think the reason it's named like that is because we wanted to change the precision of it. Maybe we should return Duration though and call it refresh_rate?

[daxpedda]

Maybe we should return Duration though and call it refresh_rate?

This actually made me look into the definition of the word "Refresh Rate" and while I couldn't find anything better quick, Wikipedia says:

... the number of times per second that a raster-based display device displays a new image.

So it would make sense to use Hz for this. The fact that we renamed it makes a lot of sense now.

Reading through #2352 also convinced me that the way it is right now is better then what I'm proposing here.