0.7.7

Changelog

• mpv now uses a dedicated socket file. This way multiple instances of
  PyRadio can be executed.
• Introducing session locking.
• Added the "--unlock" command line parameter, to force sessions' unlock.
• Added "M" command, which will jump to the middle of the list.
• PyRadio can load external theme files.
• Three more themes added. These are system themes (actual files).
• Theme selection window reworked - themes are separated by location,
  theme selection is remembered when resizing, and loading default or
  saved theme (in config window).
• PyRadio will report reverting to default theme.
• PyRadio will check and report when a new release is available.
• Added good bye message.
• Theme editor implementation started (disabled for this release).
• Minor other fixes.