Adjust layout of empty Timeline page to empty Projects and empty Assets page

Please adjust the layout/styling of an empty timeline...

... to the layout of an empty Assets or empty Projects page:

--> Place the "add asset" menu more in the center. Add arrow next to the menu and text "It looks like you do not have any entries to display" above (like on the assets and projects page). If there is an entry on the Timeline the "add asset" menu can appear where it is positioned usually.

This will close issue #896 (closed) and #904 (closed).