Summary:
* Wrote new KCM based on KQuickAddons::ConfigModule.
* Remade QMLs for Buttons and Themes tabs.
* Updated bridge model code for new plugin lookup API (fixes warnings).
* Fixed decoration shadow changing messing with the previews sizes.
* Fixed button drag and drop issues (see D18104).
* Fixed default settings button behavior and detection of settings changes.
* Updated Get Hot New Stuff.
* Removed apply button in previewbridge.cpp: After applying changes, a theme's KCModule is invalidated.

BUG: 389431
BUG: 350122
BUG: 346222
BUG: 342816
BUG: 397595

{F6574963} | {F6574962} | {F6574961} | {F6574960}

Test Plan:
* Verified saving and loading for every setting
* Checked shadows of Breeze and Oxygen
* Tested all possible drag&drop operations on both sides of the fake titlebar
* Changed color schemes (with `kcmshell5 colors`) while showing the Themes tab to see if all previews update correctly their palettes
* Tested on a fresh Neon-developer account, via kcmshell and systemsettings

Reviewers: #vdg, abetts, ngraham, #kwin, davidedmundson

Reviewed By: #vdg, #kwin, davidedmundson

Subscribers: zzag, GB_2, ngraham, broulik, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18458