Read config files in system locations before user-writable config files

This changes the order in which the global and application config files are read. In the past all global config files (/etc/kde5rc, system.kdeglobals, kdeglobals) were read before all application config files (and, on Windows, before any registry entries). This made it impossible to enforce application specific settings by marking them immutable in an application config file stored in XDG_CONFIG_DIRS (and equivalent for other OSes). One had to put those immutable settings for a specific application in /etc/kde5rc (or the system.kdeglobals file in XDG_CONFIG_DIRS) risking unintentionally enforcing a setting for other applications because settings in the global files apply for all apps and a collision between settings of different apps with same group name and key are always possible. Even worse, on Windows, users could override any settings enforced via the registry by putting the setting in one of the user writable global config files and marking it as immutable.

The new order ensures that all config files stored in XDG_CONFIG_DIRS (and equivalent) which are usually not user writable locations are read before any config files stored in user writable locations, i.e. the order (on Linux without custom XDG_CONFIG_DIRS) is /etc/kde5rc /etc/xdg/system.kdeglobals /etc/xdg/kdeglobals /etc/xdg/appnamerc [on Windows: HKLM and HKCU] ~/.config/system.kdeglobals ~/.config/kdeglobals ~/.config/appnamerc