[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:
1. Apply D27956
2. Restart kded5
3. 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
4. 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