BannerImage: Add property to control heading icon size

Joshua Goinsrequested to merge
work/redstrate/qml-bannerimage-icon-size into master

By default it uses the large icon size, which blows up certain icons compared to their text (like in the SMART Status KCM.) What I actually wanted is to use sizeForLabels, but there wasn't a public API to set the desired size of this icon.

I didn't change the default title icon size, it still defaults to the expected large icon size.

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