Pointers: support custom acceleration curves

Allow changing the length of array properties of a pointing device. This patch set removes the restriction, that array property length must match between xfconf and x-server. New buffer for the value is allocated (and freed) if the length needs to grow.

The commits cherry-pick cleanly to master, but I have not been able to test that. Therefore the MR is for the 4.18 branch.

Example of xfconf settings to get a custom acceleration curve:

xfconf-query -c pointers -p /PixArt_Dell_MS116_USB_Optical_Mouse/Properties/libinput_Accel_Custom_Motion_Step -n -t double -s 1.0
xfconf-query -c pointers -p /PixArt_Dell_MS116_USB_Optical_Mouse/Properties/libinput_Accel_Custom_Motion_Points -na -t double -t double -t double -t double -t double -t double -t double -t double -s 0.0 -s 0.19 -s 0.5 -s 0.96 -s 1.6 -s 2.5 -s 3.6 -s 5.0
xfconf-query -c pointers -p /PixArt_Dell_MS116_USB_Optical_Mouse/Properties/libinput_Accel_Profile_Enabled -na -t int -s 0 -t int -s 0 -t int -s 1