Revert "Deleted obsolete lines in importing.page, movearoundtimeline.page and usingclips.page. Added keyseq tags where appropriate. Plus some minor stuff: one link, one key shortcut."

This reverts commit 002ad9a3fbf48fcbaf68f2fe3229dc29a0ae1736.
Revert "created a tip in splitting.page; added license of pitivi itself to license.page; created grouping.page"

This reverts commit cd02807bfea07bf6f345ebf127315add7443f8b7.
ADDED LICENSE OF PITIVI ITSELF TO LICENSE.PAGE

- deleted trailing whitespaces where found