* Helper for whether a slider is vertical

* Only accumulate delta for the current slider's direction

* Slow down horizontal cursor movement while ctrl is held

* Add option 'Space bar slider sensitivity'

* Fix slider moving the cursor twice at the beginning

The slider was generating two events when first activated.
Since 2bed42857.

* Make slider less sensitive for the first step

This moves the second slider step approximately 40% further and takes the
new sensitivity option into account.
Allows moving the cursor by one spot more easily.

This was removed by mistake in 42528ab211.

Make vertical sliders slower. The intention is to make the up/down
slider slower, as we have less visibility and do smaller movements in
that direction.

The selection mode was not exited when the selection was cleared with,
for example, ctrl+x.

Emptying the selection with the spacebar slider puts the selection mode
in an unexpected state. This checks that this doesn't happen.

The selection ends can now cross each other, reversing the selection
order. Special care is made to make sure that the sliders go in the
expected direction.

Pointers is changed to send a `onPointerDown` for the first slider event
instead of a `onPointerHold`. This event is detected in
`KeyEventHandler` and the selection ends are reordered if needed when
sliding again.

The distance at which a swipe was considered successful was reduced in
188c682 to allow making circle gestures on a key that also had a slider.
It was also useful in reducing accidental sliding.

This is impractical for normal typing so it is reverted. The reduced
range continues to be used for sliders.

This allows starting an anticircle gesture on a key with sliders on two
opposite sides.

This is especially useful when discovering the circle gestures and to
feel when a gesture has been entered by accident.

Sliders are no longer limited to the x dimension and can be placed in
corners or purely on the y dimension.