Close KSplash when the desktops are visible

Summary:
If the desktops are already visible, there's no much need to hold on the KSplash. Panels should be ellegantly displayed when they're ready.

As discussed in T10958.

Test Plan:
Made sure that it gets emitted at the right time.
Timing-wise, my system takes about 1s to show the DesktopView and 2s to show the PanelView (I have 1 panel)

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: broulik, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21494