bzr2 loading dlls from other installations

[ciros88]

when running freshly unpacked/compiled bzr2 instance with a vital dll (like qt or plugin) remove, it still works.
this seems due to the presence of a bzr2 installation on the system.
this means there is an unwanted correlation between the running instance and any bzr2 installation: we should check if app dlls are specifically load from the running instance's dir.