Add a widget gallery page to the Dashboard.

Summary:
This is one of the tasks on the Plasma 5.9 release todo, as
agreed at the kick-off meeting. Dashboard is about enabling
an alternative fullscreen workflow for people who want one,
and this extends the coverage to widget management. It's also
a widget management workflow many people are used to from
their phone.

This is quite early code, but already works fairly nicely.
There's even polish like pre-loading the widget explorer model
as soon as the tab is hovered to speed up the tab switch, and
keyboard nav is working, too.

I would like to get it reviewed now and merged once egregious
technical founds are identified and eliminated. Please don't
be too picky on the visual or even workflow details - one
reason I want it merged early is so that I can get it into
the VDG's hands via Neon for advice, and also because I want
people to spend some time using it in general. It's early
enough in the 5.9 cycle to iterate more.

In addition to the attached screenshot, here is a video demo:
https://www.youtube.com/watch?v=ajIzfU0eJtI

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3200