matroskademux: make max backtrack distance for keyframe search configurable

matroskademux: make max backtrack distance for keyframe search configurable

Add property instead of hardcoding it in the code.

In some scenarios such as CCTV variable fps and extra long GOPs are
used to minimise storage space, for example. In those cases there might
not be any keyframes for many minutes, so provide a property to override
the max allowed distance.

https://bugzilla.gnome.org/show_bug.cgi?id=790696