Revert "remove IDE-specific files (stage 2)"

This reverts commit df2ae75e2771d70c35ebfcab87448057d76733eb.
Revert "remove IDE-specific files"

This reverts commit 104a8f0fe3a4c47305a9df57d6f42f42317bf6a3.
Revert "gitignore IDE-specific files"

This reverts commit 38c532eef2a4826618f3c277af8be40fed1c8f30.