Simplify and enhance Windows installer

- Add icons to sclang and scide
- Simplify installer-script and remove unused code
- Register installer in Add-/remove programs
- Associate filetypes scd, sc and schelp with scide
- Add startmenu item
- Brand installer with cube icon
- Remove registry settings on uninstall
- Move icon-location and enhance relative path-logic to prevent build-error with build folders which are not subfolders of the SC-source folder (RelPath cleanup)
- OT: Add two file-types to gitignore 

Remark: I messed up my previous pull request by merging in master. This is a new branch created by cherry-picking the previous commits and adding a few tiny enhancements concerning pathlogic (RelPath cleanup)