Re-arrange toolbar buttons by usage frequency

When using an LTR language, people scan user interfaces left-to right
and top to bottom. This means that the most frequently-used controls
should be located at the top left, so people see them first.

This commit uses that principle to re-arrange the toolbar a bit, so that
after the "show/hide sidebar button" (which needs to be the left-most
item to connect it with the left sidebar), the left-most items in the
default toolbar are the zoom and view mode controls. These are
viewing-related controls, and Okular is first and foremost a viewer app.
The controls that let you interact with the document (select stuff,
annotate) are moved over to the right side of the toolbar because they
are used for secondary workflows.

The zoom buttons are also grouped together to connect them in the user's
mind.

Finally, the separator lines are re-added to disconnect the "show/hide
sidebar" button from the now-icons-only zoom buttons.


(cherry picked from commit 8c8744f2)