Support global defaults for input devices
On Wayland, KWin is responsible for the configuration of input devices. In certain cases, it is useful to be able to provide defaults for this configuration. This adds support for reading a "Defaults" section from kcminputrc that is used as default over the libinput defaults, which are hardcoded. This allows setting a different acceleration profile for all devices, for example.
The first commits does some refactoring of the ConfigData code in the libinput Device class, the second implements the new defaults.