[voicecall] Port to Qt 5

libvoicecall, and voicecall-manager are Qt 5-only after this commit, as are
all plugins except the declarative plugin, which remains to allow Qt 4-based
UIs to communicate with the Qt 5 manager process.
[voicecall] Rename uri for consistency with the rest of the platform.
Merge pull request #12 from rburchell/qt5port

[voicecall] Port to Qt 5