Switch to using Kirigami's ShadowedRectangle

Summary:
This switches from using the QML DropShadow to using the
new, high efficiency ShadowedRectangle in Kirigami.

• Modify (and equalise logic) in GridTileDelegate
• Clean up EntryScreenshots, and use ShadowedRectangle
• Also use ShadowedRectangle in the bigpreview and tile delegates

Test Plan:
No visible changes (as expected) from the switch
Minor visible change from the EntryScreenshots modifications (now correctly shows the shadows all the way around)

Screenshots component without changes:

Screenshots component with changes:

Reviewers: KNewStuff, Frameworks, Plasma, ahiemstra, broulik, mart, VDG

Reviewed By: Plasma, broulik

Subscribers: davidedmundson, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28220