ktooltipwidget: Fix tooltip positioning

Summary:
Partially fix Bug 388583
Display tooltip at correct position in dolphin when

• Tooltip is displayed above file item, or
• Content width exceeds screen width

CCBUG: 388583

Test Plan:
Visual inspection specially for equal distance to file item (3px)
Patch applied:

Currently partial display:

Patch applied:

Currently invisible tooltip (displayed off-screen).
Patch applied:

Reviewers: elvisangelaccio, ngraham

Reviewed By: elvisangelaccio

Subscribers: Frameworks, dfaure, cfeck

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9973