Set central splitter to non collapsible

Summary:
The sidebar can be toggled by {key F4}, {nav View > Sidebar} or
a button in the status bar. Hiding the sidebar by dragging the
central splitter to a collapsed state may confuse users if they
can't toggle the sidebar anymore using the normal ways.
This gets even more awkward saving the splitter state in D12489.
This patch simply disables collapsing for the central splitter.

FIXED-IN: 18.08.0

Test Plan:
* Open Gwenview with visible sidebar
* Move the horizontal splitter left and right
* It should not be possible to collapse one side

Reviewers: #gwenview, huoni

Reviewed By: #gwenview, huoni

Differential Revision: https://phabricator.kde.org/D12561