Added an ability to cancel the currently running strokes!

You can either press Esc key or press a red cross button which appears
near the progress bar in the bottom. The currently executing stroke will
be cancelled and all its work will be reverted.

This patch will first go to 'progress-bar-reincarnated-kazakov' branch.
Please test it for stability and usability please!

CCBUG:252071
CCMAIL:kimageshop@kde.org