Separate builds for ICU

Add init job for ICU builds (latest/snapshot directory
structure) and actual build handler for ICU builds.

The first version includes Linux builds only.

Change-Id: If620d45e4dff029627434ade23f0f37a05c7b3fe
Reviewed-by: Jani Heikkinen <jani.heikkinen@theqtcompany.com>