Backend AutoDiscovery improvements

[Nigel]

1. If a valid mysql.txt exists, the backend selector should not appear

2. The selector should be use MythTV widgets (colours, theme, et c.). This should also automatically add LIRC, Apple remote, and joystick support.

3. Any Myth program should be able to select a backend, not just the frontend

4. The programs could autoselect if only a single backend is available (and probably not save the backend UUID in config.xml, or even generate one). I am still thinking about this, but am hoping this would allow a bootable CD frontend to work without having to save any config anywhere

Attached patch implements 1-3, but currently the PIN dialog is annoying, because it uses the SMS-style button pressing. Please test

[Nigel]

Initial version. Requires mediarenderer.* and masterselection.* to be moved into libmythupnp