[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:
- `Settings.ini` uses new booleans (`true` and `false` instead of `1` and `0`)
- `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