Fix memory leak in system tray

Summary:
Currently we leak an IconLoader for each icon for the time the system
tray is in memory.

An app may also load icons from their own directories, so we need a new
iconloader that takes this into account

This is wrapped into a subclass of iconengine so the iconloader lifespan
matches the icon object

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1498