[Icon KCM] Port to new design

This completely drops the icon customization with icon effects and only retains the icon size settings.
Should there be any custom icon effects, pressing "Defaults" in the KCM will remove them.
I also couldn't find any trace of that "animations enabled" check box it used to have.
It adds drag and drop support for installing archives as themes. It can also download them from remote location
when dropped from e.g. a browser.

Installing theme files actually never worked in Plasma 5 as it got broken in the port from KIO NetAccess
to storedGet.
An animated preview is also added on hover showing a common selection of icons.

Implements T7262

BUG: 367619
BUG: 334301
CCBUG: 163992
FIXED-IN: 5.13.0

Differential Revision: https://phabricator.kde.org/D12459