* gtk: add cursor theme configuration
- Added the `cursorTheme` under the gtk module.
- Added tests for the gtk3 settings file generation, and renamed
the gtk2 unit test expected file for clarity.
- Added guard against generating a blank `gtk.css` when `cfg.extraCss`
is empty.
- Replaced `concatMapStrings` calls with `concatStringsSep`. The library function
`concatMapStrings` generates an intemediate list which is then passed to
`concatStringsSep`, As we are not performing other transformation except
the addition of newlines, a direct call to `concatStringsSep` is sufficient.
- Updated description of examples to be more general "~/.config" -> "$XDG_CONFIG_HOME".
- Update helper functions `toGtk3Ini` and `formatGtk2Option` to use the library
function `boolToString` and escape the separator in the key name.
* xcursor: delegate GTK cursor settings to gtk.cursorTheme
- Added deprecation warning for GTK settings in the `xsession.cursorTheme` module.
- Modified config section to use `gtk.cursorTheme` for GTK cursor settings.
This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
