build: i18n: windows: first attepmt at building tools

doing it wrong, via git
build: i18n: windows: able to projgen
build: i18n: windows: full-icu working!

Code is able to build a complete ICU now.
build: i18n: actually, works fine on linux. Didn't manage to break it.
build: i18n: windows: build iculslocs

needed for manifest rebuild of ICU data
build: i18n: windows: enable "small-icu" build

also update docs

still very much based on version number and a "special ICU branch".
Merge pull request #2 from srl295/smaller-icu-windows-data

build: i18n: This adds icu-small and icu-full options to the vcbuild.bat script. note README.md updated about dependencies.