Add Avogadro GL Widget to MongoChem

Enable 3D structures to be displayed in GL Widget.

Change-Id: I266d2ed1381fa7fb91252f587c5386daf5f4ae97
Remove 3D tab if no structure is available

Change-Id: Ic7ed8941d20d667d65588d40ac4a83d835087ae1
Added REQUIRED and NO_MODULE to find_package calls

Make it clear we are expecting a Config.cmake file for VTK and
AvogadroLibs, and fail here if they are not found.

Change-Id: I4456faf2dee150cf189e87cc6e6b26a59d2e145c