libtaskmanager: Simplify ScreencastingRequest implementation

Vlad Zahorodniirequested to merge
work/zzag/missing-window-thumbnail into master

The d pointers are not required because the ScreencastingRequest is exposed only to QML.

ScreencastingStreamPrivate::~ScreencastingStreamPrivate() has another issue - it calls q->deleteLater(), which seems dangerous.

Edited by Vlad Zahorodnii

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