[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
[voicecall] Don't pretend that we succeeded to call something we don't know about
[voicecall] Fix compilation with debug enabled.
[voicecall] Fix error passing from providers.

Providers were successfully checking for errors and emitting them, but the
manager ignored their plaintative cries for help and did absolutely nothing,
so the UI never heard about their plight and did nothing while the user
wondered why their phone was not making the ordered phone call for 5 minutes
before throwing it out the window.

Errors are still rather broken:
1) they are not i18n-friendly (hardcoded english)
2) they are not user-friendly ("org.freedesktop.Telepathy.Error.Disconnected:
   Channel aborted")

but at least they are passed up successfully.
[voicecall] Optionally enable Telepathy debug output if WANT_TRACE is defined.
[voicecall] Correct debug output in the error case for dbus pending call watcher.
[voicecall] Remove dependency on xorg.target from voicecall-manager.

This won't work in a Wayland-world, and we also don't require anything
graphical, so we can start whenever.

This assumes that voicecall-ui implementations have a requirement on
voicecall-manager.service.