Use clean overlay mechanism for build

[twagoo]

Currently the build script selects a few files from the sites/all/themes/CLARIN_Horizon directory and adds them to the target directory, overwriting some default files (towards the end it also adds variables.less from the base style project):

(cd 'sites/all/themes/CLARIN_Horizon/'
mv -f -- 'CLARIN_Horizon.info' 'favicon.ico' 'logo.png' ~/'CLARIN_Horizon/')

The rest of the sites/all/themes/CLARIN_Horizon directory content is ignored. This obfuscates the development and build process to a certain degree. A better approach would be to make a pruned version of this directory a complete 'overlay'.