Keymaps: add Russian ruble symbol...

to Russian, Belorusian and Ukrainian keymaps

Signed-off-by: Adrien Destugues <pulkomandy@gmail.com>

This is the new official symbol for ruble since october 2013, and was
added to Unicode 7.

Note that our version of the DejaVu font doesn't support it yet, we need
to update it as well.
Minimal documentation for BOptionPopUp.

This was available in R5 but somehow went missing from our version of
the Be Book (it is documented in the release notes in Tycom Systems
version). Document the more important methods of it, at least.
MixerToy: app for testing mixer resamplers.

* Shows a source wave (black), a sampled version of it (red), and a
resampled version of that using the mixer (green).
* Debugging tool for #9438. Shows that the drop/repeat resampler is
working ok, but interpolate is quite broken.