kded: update `scaling-factor` when setting global scale

Some applications like IntelliJ IDEA will read
org.gnome.desktop.interface/scaling-factor to get the system scale factor.
While the behavior is questinoable, the workaround is simple and
harmless so users will not need to find a workaround on the Internet
or blindly blame KDE and switch to GNOME instead.

Reference: https://intellij-support.jetbrains.com/hc/en-us/articles/360007994999-HiDPI-configuration

(cherry picked from commit 53aa07cf)