Fix for SUSE L3 bug 624546 (freedesktop 83733)

Problem:
When I'm in outline mode, if I select a slide in the left slide preview
pane, ctrl-c, then select another slide I would like it after and hit
ctrl-v; it refuses to cut/paste.

Analysis:
This issue is due to the fact that the outline view always grabs focus
when activated and a view is activated whenever is pushed to the
sfx shell stack even if it is not the new top-most active view shell
(see `ViewShellManager`, `SfxViewShell`, `SfxDispacther`).

Solution:
Make the `OutlineViewShell` grabs focus only if it is the top-most
active view shell.

Rationale:
When `OutlineViewShell::Activate` is invoked, instead of removing
the focus grabbing action completely, we check if the
`OutlineViewShell` instance is the the top-most view shell and
perform the focus grabbing action only in such a case. This change
required to have also the `DrawViewShell` grabbing focus on
activation (only when it is the top-most view shell). In order to
implement this solution I needed to add a new method (and data
member) to the `ViewShellManager` class. I named it
`GetTopViewShell`. This method returns a pointer to the top-most
active view shell of the internal stack. The returned pointer is
updated in the `UpdateShellStack` method, before the sfx shell
stack is updated.

For more details see :
https://gist.github.com/mcecchetti/15b3ebc505d6582ea0db

Change-Id: I619a406864f50f0e62dee3fcb9ac5d46e3d48272