Currently the Makefile lets you choose your own PREFIX, but
the installed runanki.system always hardcodes the prefix as /usr.
Fix runanki.system at install time to install into the designated
PREFIX.
- a few issues to work out still, and editor changes not done yet
- for communication between webengine and python code, we set window
.location to 'http://anki/<something>' - the leading http is
necessary for qt to call the link handler, which was introduced
in qt5.5
- the designer files now use a promoted qobject to create instances
of AnkiWebView
- we use the css zoom property to alter webengine font size based on
system dpi
- prefs and addons folder stored in new location (at least for now)
