remote-desktop: Allow using custom scroll source for NotifyPointerAxis

Quoting the commit message here:

Currently, the NotifyPointerAxis method always assumes that the scroll
source is CLUTTER_SCROLL_SOURCE_FINGER.
This is however not always true and in some cases a remote desktop
client needs to submit a PointerAxis event with a custom axis step.
This is for example the case with high resolution mouse wheels, where
the NotifyPointerAxisDiscrete method is unsuitable.
In such cases NotifyPointerAxis needs to be called, but with the
intention that the scroll source is still a mouse wheel.

To solve this situation, don't assume the scroll source always to be
CLUTTER_SCROLL_SOURCE_FINGER.
Instead, add further flag options to NotifyPointerAxis, which allow a
remote desktop client to choose the scroll source.
This way a remote desktop client can choose what scroll source is the
most suitable one for the current scroll event.

Intended to work together with gnome-remote-desktop!25 (merged)