Add support for custom pointer acceleration

This patch adds support for the new custom pointer acceleration method in libinput, see libinput/libinput!775 (merged).

Adds new properties and xorg.conf entries for setting the acceleration function's points and step.

AccelProfile option can now accept custom value.

Add 4 new options which only apply when AccelProfile is custom:

• Add AccelPointsFallback option for setting the points of the Fallback acceleration function. Points values are represented by a space-separated list, e.g. "0.0 1.0 2.4 2.5".
• Add AccelStepFallback option for setting the step of the Fallback acceleration function. When a step of 0.0 is provided, libinput default Fallback acceleration function is used.
• Add AccelPointsMotion and AccelStepMotion options, which are equivalent to AccelPointsFallback and AccelStepFallback options, but apply to the Motion acceleration function.

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Based on !38 (closed).
Have been using the new custom profile on my machine for a while now, awaiting public testing.