fdo#72420: Startcenter: make all helpid point to startcenter's main help page

Until the specific ui elements don't have own help pages.
With this we can avoid to open an empty help page by pushing F1 or
clicking on Help button.
Only two ui element's helpid is not set yet: 'Extensions' button
and recent docs view.

Change-Id: Ic0c1bd1dba4c08fd6b3d9daf8fbc9e622a54b6f2