Notable changes:
* Use toml as new default storage backend:
  Upstream has removed the ini plugin, we have been using in the
  past. toml is still somewhat experimental, but upstream has
  designated it as future recommended default.
* Remove the dependency on boost. The only plugin needing that has
  been removed upstram.
* Enable plugins for libev and libuv event loop integration. This
  adds two new packages with the respective dependencies.
  Upstream has fixed the bugs preventing us from using this.
* Enable the internal notification plugin/system. This allows
  applications to automatically receive notifications about changes
  to their configuration. Again upstream fixing bugs enables this
  for us.
* Set the environment variable $XDG_CACHE_HOME to "/tmp/" globally
  to prevent elektra (and possibly other applications) from
  writing temporary data to flash.

Signed-off-by: Harald Geyer <harald@ccbib.org>