Multi-finger gestures should continue when one finger lost when possible

[Michał Sawicz]

Unity8 uses 3-finger gestures for dragging windows, but if any of those fingers is lost, the gesture is interrupted. Instead, we should accept the three fingers and only interrupt the gesture when all three fingers are lifted. This way the drag gesture should be much more reliable.

An additional question is whether we should re-grab the fingers within the area they are expected (so e.g. over the window being dragged) - this would then allow you to continue to drag when one of your fingers went out of the touch area and got back in.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: unity8 8.12+16.04.20160620.3-0ubuntu1 [origin: LP-PPA-ci-train-ppa-service-stable-phone-overlay]
ProcVersionSignature: Ubuntu 4.4.0-24.43-generic 4.4.10
Uname: Linux 4.4.0-24-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Jun 27 17:53:32 2016
InstallationDate: Installed on 2016-05-06 (51 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: unity8
UpgradeStatus: No upgrade log present (probably fresh install)