Add a visual indicator to memory reporting.

This turns the memory reporting button into a slider that fills up with color
as the memory usage increases.

* From 0% to 20% it'll be the themes' highlight color.
* From 20 to 40% it'll mix to turn towards a dark yellow.
* From 40% to 80% it'll mix to turn towards a darker red.
* From 80% onwards it'll be red.

This should help less technical users determine whether their computer can handle what
they're requesting it to do.

CCMAIL:kimageshop@kde.org
Differential Revision: https://phabricator.kde.org/D19167
Ref T10368