REMOVE OBSOLETE PARTS AND MINOR UPDATES

- the whole Getting Started part was reviewed
- changes made to importing.page, movearoundtimeline.page and usingclips.page
CREATE NEW PAGE EXPLAINING GROUPING

- new page grouping.page
- add the new page to Makefile.am
ADD KEYSEQ TAG WHERE APPROPRIATE

- add keyseq instead of <key>key</key>+<key>key</key> in movearoundtimeline.page
- add keyseq instead of <key>key+key</key> in cheatsheet.page
DOCUMENT CREATING NEW LAYERS

- document the procedure of creating new layers
- fix bug 720878