kcms/energy: Use a card background for the graph

Ismael Asensiorequested to merge
work/iasensio/energy-graph-card into master

Use a ShadowedRectangle as the background around the whole graph, instead of custom painted one just for the plot area.

As it was suggested in !227 (comment 1130560)

BEFORE AFTER
Breeze Light energy-card-before-light energy-card-after-light.png
Breeze Dark energy-card-before-dark energy-card-after-dark.png
Custom energy-card-before-custom energy-card-after-custom.png
Edited by Ismael Asensio

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