Add support for resizing by dragging the title bar

Summary:
This is for the benefit of users who find changing the window height
through the configuration window overly tedious.

This may cause slight flickering during resizing.

FEATURE: 154686
FIXED-IN: 20.04.0

Test Plan:
Run patched Yakuake.
Drag Titlebar up and down the screen.

Yakuake should automatically update its height configuration while being dragged.

Reviewers: Yakuake, hein, mweepigeon, ngraham

Reviewed By: mweepigeon, ngraham

Subscribers: mweepigeon, ngraham

Tags: Yakuake

Differential Revision: https://phabricator.kde.org/D22227