[mythtv-cmyth] Release 1.6.8

Hi Lars,

this is an update for the MythTV addon (to version 1.6.8).
The most noticeable changes are the huge performance increasements in all areas.
This makes the addon useable on low end hardware like the Raspberry Pi.

We have tested this (with the help of some forum users) for quite a while now on Linux, Windows, MacOS and RPi.

A backport to Frodo would be helpful and desirable because many issues that people are reporting
in the forums are already resolved by this.

Be warned: it's big and might block your browser for a while (due to a refactoring in libcmyth)

Thanks,
Christian

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Changelog:

v1.6.8
- Performance improvements
    - Reduced startup time (Faster loading of EPG, channels, recordings and images)
    - Reduced resource usage for image caching
    - Cache recording data (event triggered updates from backend)
    - Fixed lock when stopping recording playback
    - Improved general response time
- Live TV improvements
    - Better handling of connection problems
    - Playback does not stop anymore between program breaks
    - Fixed seek problems
- Timer improvements
    - Add optional internal recording rule template
    - Correctly apply backend recording rule settings
    - Properly add title and subtitle
    - Fixed Timer deletion
- Added advanced option for specifying database host/port
- Fixed crash when loading invisibile recordings
- Improved Art-work loading
- Combine channels with same channum and callsign
- Fixed IPv6 support
- Fixed Windows XP support
- Improved logging, notifications and system labels
- Refactored & cleaned up libcmyth (inttypes, names, docs)