Complete TextSelectorEngine if nothing selected

Selecting highlight tool, pressing left mouse button while over text, and
immediately releasing without dragging a selection caused inconsistent state.

It left TextSelectorEnigne in the state m_creationCompleted == false and
m_lockedItem == something. Then in continuous mode all events kept on being
propagated to TextSelectorEngine::event, even if the user started unrelated
interactions in the meantime. This caused various side effects.

It notably happened when you double clicked a finished highlight annotation,
as described in bug 426658.

We can go to m_creationComplete after release even without a selection,
because TextSelectorEngine::end and PageViewAnnotator::performRouteMouseOrTabletEvent
handle the case just fine, i.e. don't create an annotation but reset the state machine.

Fixes bug 426658 at least partially. I couldn't reproduce the described crash,
so no idea if that's also fixed.

BUG: 446658
5 jobs for !291 with bugfix/textselectorengine in 47 minutes and 33 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Build
passed #72248
build_clazy_clang_tidy

00:22:03

passed #72246
build_ubuntu_18_04

00:21:19

passed #72247
build_ubuntu_20_04

00:25:42

passed #72249
clang_format

00:02:05

passed #72245
linux kf5-qt5 qt5.12

00:13:02

44.0%