[GTK Config] Port all settings to KConfigWatcher
Summary:
Also remove redundant reparse configuration calls and change
the ways of reading some settings accordingly.
Depends on: D27956
Test Plan:
- Apply D27956
- Restart kded5
- Apply all the settings possible:
- Fonts
- Icons
- Color Scheme (for dark theme preference setting)
- Icons on buttons
- Icons in menus
- Toolbar style
- Scrollbar behaviour (what happens if you click on scrollbar)
- Animations speed (though I haven't touch it in this diff)
- Decorations button order
- Check if all the settings are correctly applied to GTK applications
Reviewers: Plasma, ngraham
Reviewed By: ngraham
Subscribers: ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D27957