[GTK Config] Parameterise GTK config editing

Summary:
This diff is only touches the editing part of the ConfigEditor.

Changes:

1. Pass config values as the respecting types: int, bool and string. This simplifies config editing in some extent - no more hardcoded options names, that requires quotes around values in gtkrc-2.0 and xsettingsd.conf. Also it simplifies working with GSettings API and helps to transform KDE config options to GTK config options.
2. Remove redundant \\n symbols, when adding new option in gtkrc-2.0 and xsettingsd.conf

Test Plan:

1. Restart Kded
2. Test all settings syncing. Specifically boolean ones and enum ones, as the format in configuration files has changed:
3. Settings.ini uses new booleans (true and false instead of 1 and 0)
4. Settings.ini, .gtkrc-2.0 and xsettingsd.conf use int value of enum instead of string

Reviewers: Plasma, cblack

Reviewed By: Plasma, cblack

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27100