Apply the correct palette to icons

Summary:
Our icons can be recolored. However there is a difference between custom colors
on widgets and icons. We will respect the palette but KIconLoader that creates
the icon pixmaps operates on an application wide palette basis. This can create
miscolored icons when a widget has a custom palette. A helper function is
introduced to load the pixmaps that switches the palette of the global
KIconLoader if necessary and resets it appropriately.

Test Plan:
Before:
{F8205856}
After:
{F8205857}

Reviewers: #breeze, ndavis, cblack, hpereiradacosta, mart

Reviewed By: cblack, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28433