dvbsrc: make slof/lof1/lof2 settable properties

dvbsrc: make slof/lof1/lof2 settable properties

Allows proper tuning around high/low band boundaries when using
non "standard" LNBs.

Not all LNBs (Low noise block down converters) are made equal.
This is particularly true for universal LNBFs, where, even though
there are seemingly standard values for the local oscillator
frequencies, these can vary from manufacturer to manufacturer
and LNB model. Change also proxies the new LNB properties in
dvbbasebin.

https://bugzilla.gnome.org/show_bug.cgi?id=732818