Batch window changes events on XCB

Summary:
In a log from someone talking about high CPU we can see get multiple X
events for the same window as multiple events, but directly next to each
other. This causes the TaskManager to process changes multiple times
instead of just once which is a waste.

An example is just pressing "enter" in konsole, which will pointlessly
update the title.

This causes problems for expensive app lookup and also QML performs text layouts immediately so any text changes cause quite large CPU usage if done more than 60fps; especially a task text resizing
could result in resizing the entire panel.

Something not relevant in kwin that also monitors these rolls.

This class sits between KWindowSystem and XWindowTasksModel
transparently buffering the changes.

CCBUG: 378010
BUG: 365317

Reviewers: #plasma, hein, broulik

Reviewed By: #plasma, hein, broulik

Subscribers: ngraham, cfeck, broulik, hein, graesslin, plasma-devel

Tags: #plasma

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