libshared: Change string_for_rate to use KiB/s instead of Kbps.

string_for_size uses KiB, etc., and so when the two are combined (e.g.
pkgman's progress display), it looked especially strange to have two
different units.

StringForRate(): adjustments

* Make 1000.0f the default base
* Also use sizeof() in MediaPlayer to get the string size rather than hardcode.

Thanks Marcus

StringForRate()

Introduce a function to generate the string representation of a bitrate
(kbps, mbps, gbps, etc..)

* Factor out the code from MediaPlayer InfoWindow
* Allow different bases (/1000 or /1024)

StringForRate(): adjustments

* Make 1000.0f the default base
* Also use sizeof() in MediaPlayer to get the string size rather than hardcode.

Thanks Marcus

StringForRate()

Introduce a function to generate the string representation of a bitrate
(kbps, mbps, gbps, etc..)

* Factor out the code from MediaPlayer InfoWindow
* Allow different bases (/1000 or /1024)