[packaging] Add Qt5Gui to build requirements

It's needed by ut_mdconfgroup
Merge pull request #14 from foolab/dconf

[packaging] Add Qt5Gui to build requirements
[mlite] Fix assert while resolving properties.

Bindings evaluation can result in a child being resolved
while properties for the parent are being read. Don't resolve
the child's properties a second time if that happens.
Merge pull request #15 from adenexter/mdconfgroup-assert

[mlite] Fix assert while resolving properties.
[dconf] fixed MGConfItem::listDirs()