install icons, desktop shortcuts and launchers in Makefile

avoid redundancy with package build scripts
add uninstall rules
separate RALint tests from regular builds
rename RALint → OpenRA.Lint
spaces to tabs
update documentation
apt-get update in before_install
separate Linux only shortcuts and icons from make install
Merge pull request #3837 from Mailaender/makefile-fixes

Use the Makefile install rules for our packaging