[buildicu] Windows ICU download support + touch-ups

Signed-off-by: Jarkko Lehtoranta <jarkko.lehtoranta@jolla.com>
[buildmaster] Use the "next" and "jolla-1.6" branches

Signed-off-by: Jarkko Lehtoranta <jarkko.lehtoranta@jolla.com>
Merge pull request #7 from sailfish-sdk/next-dev

[buildscripts] Windows ICU download support + change build branches