Add support for XWayland client scaling

This is very important e.g for Chromium or Electron apps.

This MR does the following:

* Renames the `ConfigValueProvider::globalScaleFactor()` method to `x11GlobalScaleFactor()`
* Removes the `ConfigValueProvider::globalScaleFactorAsPercent()`, `ConfigValueProvider::globalScaleFactorFloor()` methods as those are now useless
* The global scale factor for XWayland apps is read from `kwinrc` under the `Xwayland` group.
* Read the base `Xft.dpi` resource value from the `kdeglobals` config file

BUG: 465733
FIXED-IN: 5.27.1


(cherry picked from commit a2e0cb9f)