misc: implement update system

- re-introduce a genericized NetworkManager
- isolate update logic into a qml accessible class derived from
  NetworkManager
- fix QtWebEngineProcess missing when re-installing over existing
  version
- provide a command line option to override the base url
  for testing local and remote updates
- clean-up manual update-check UI

Gitlab: #101
Change-Id: I9c8d2badae59ec31cab12d38b8470edf2bcad401