Fix search field focus, both initial and on-demand

Summary:
Right now the search field doesn't get focus initially, or when the user types on a page.
This is because the conditional loader that conditionally loads the GlobalDrawer's
toolbar containing the search field isn't able to pass on focus or signals to its content
properly. I tried various methods of fixing this by adding `focus: true` to all the focus
stopes in the chain, but was unsuccessful. So this patch implements the conditional
loading of the top content differently: instead the top content is always loaded, but
conditionally discarded when in non-widescreen mode. This fixes the aforementioned issues.

However it may not be the correct way to fix the problem; if not, assistance would be
appreciated.

BUG: 413407
FIXED-IN: 5.18.0

Test Plan:
- Search field has focus when Discover is launched
- Search field gains focus and accept text when typing anywhere
- When the window is made mobiley, the sidebar's toolbar disappears
- When a mobiley window is made desktoppy, the sidebar;s toolbar re-appears

Reviewers: #discover_software_store, apol, davidedmundson

Reviewed By: #discover_software_store, apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24935