Create channels with m_bEPGEnabled set to false for hidden channels

When first creating a channel, set m_bEPGEnabled to !channel.bIsHidden.
This enables the EPG scanner to only work on visible channels as
default, thereby reducing scan time, especially important on setups with
thousands of available channels, but with only a limited set visible.
Enable and disable m_bEPGEnabled when changing channel visibility
Merge pull request #2288 from dodoadoodoo/master

Create channels with m_bEPGEnabled set to false for hidden channels