Qt5OpenGL is only required with optional AvogadroLibs support

Summary:
Add missing search for Qt5Xml
Add missing Qt5Widgets to compoundviewer target_link_libraries
Remove Qt5OpenGL from libscience target_link_libraries, not used

Test Plan: Built fine with multiple cmake option combinations.

Reviewers: #kde_edu, aacid

Reviewed By: aacid

Subscribers: aacid, kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D18312