Restructuring (plugin scheme, pyz package, gtk state properties)

The upcoming release 2.1.5 will be slightly restructured.

   * Plugin meta data is now utilized fully. Configuration data and built-in .title attributes will no longer reside in plugins as code.

   * Channel/plugin lookup code will use introspect/pkgutil. Long term ST2 may become a proper Python package in itself (to avoid a fixed global path).

   * It's planned to package it up as standalone PYZ (Python ZIP) application.  (As alternative to DEB/RPM builds). GtkBuilder loading and banner image need some work though.

   * State variables (.current_channel, .channel_names, .channel.current, .current_row, .default, .row) will partly be replaced by direct Gtk properties or lookups. In particular after initialization the mapping hasn't always be reliable.