Prepare for ICU update

- libicule and libiculx do not exist anymore in newer ICU versions
(harfbuzz replaces them), but we didn't actually use them, so remove
them from the build feature and from the package dependencies
- Add namespace usage marcos since the newer ICU packages put ICU things
in a namespace, making it easier to have multiple versions of ICU used
side by side.

No functional change intended, but this makes it possible to build the
code with either ICU 57 (for gcc2) or 66 (for other architectures).

Implement a formatter for relative dates.

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

It can give results such as "in 2 hours", "2 days ago", etc.
This is different from BDurationFormat which will just say "1 hour, 2
minute and 36 seconds"