[kcmkwin/kwindecoration] Rewrite the KWin decorations settings as a ConfigModule
- 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.
- 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
Subscribers: zzag, GB_2, ngraham, broulik, kwin
Differential Revision: https://phabricator.kde.org/D18458