shannon entropy to guess monochrome icon

Summary:
since we can't directly access the svg file, when we can't use
KIconLoader try to detect monochrome icons on the actual image
with the following heuristic:

• less than 30% of pixels with high saturation
• the Shannon entropy of the distribution of gray values of the

non transparent pixels is less than 0.3
this seems to give satisfying results with most icon temes

Test Plan: tested with breeze, oxygen adwayta and some other themes from ghns

Reviewers: Kirigami

Subscribers: ngraham, GB_2, ndavis, VDG, cfeck, davidedmundson, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D19392