Summary:
There are two types of constraints supported:
1. Pointer confinement
2. Pointer locking

In the case of confinement the pointer is confined to a given region of
the surface. This is comparable to general operation where the pointer
is confined to the screen region.

In the second case the pointer gets locked. That means it cannot move at
all. No further position updates are provided, only relative motion
events can go to the application. There is a hint about cursor position
update on unlock which is not yet implemented in KWayland::Server, thus
also not in this change.

The implementation in KWin grants the requests for pointer constraints
when the pointer enters the constrained region, either by pointer
movement or by e.g. stacking order changes. There is no confirmation
from user required to enter that mode. But we want to show an OSD when
the pointer gets constrained, this is not yet implemented, though.

Breaking an active constraint is relatively easy. E.g. changing the
stacking order will break the constraint if another surface is under the
cursor. Also (in case of confinement) moving the pointer to an
overlapping window breaks the confinement. But as soon as one moves the
pointer back to the window a constraint might get honoured again.

To properly break there is a dedicated event filter. It listens for a
long press of the Escape key. If hold for 3sec the pointer constraint is
broken and not activated again till the pointer got moved out of the
window. Afterward when moving in the pointer might activate again.

The escape filter ensures that the key press is forwarded to the
application if it's a short press or if another key gets pressed during
the three seconds. If the three seconds way fires, the later escape
release is not sent to the application.

This basic interaction is also ensured through an added auto test.

This change implements T4605.

Test Plan: Added auto test and nested KWin Wayland with D3488

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3506