Lazily load file entry icons

Summary:
Building the icons datastructure is the most significant bottleneck when opening large archives. Knowing all the icons at loading time is unnecessary since most elements won't be visible anyway.
This patch moves the icon storing into the Entry class where it is lazily determined the first time it is accessed. This improves the initial archive loading time significantly.

Test Plan: It's hard to give exact numbers, but my main test archive (the Android NDK) went from ~3 seconds to load to "pretty much instant"

Reviewers: #ark, elvisangelaccio

Reviewed By: #ark, elvisangelaccio

Subscribers: broulik, kde-utils-devel

Tags: #ark

Differential Revision: https://phabricator.kde.org/D26357