filesrc: don't try to seek to -1 offset

The offset can be -1 when we are configured in TIME format. Instead of
failing the seek and erroring, do what and offset of -1 is supposed to
do and simply read from the current offset.
multiqueue: Allow growing a queue if all other queues are not linked

See https://bugzilla.gnome.org/show_bug.cgi?id=719893
info: debug segment offset field as well
collectpads: take offset into account for expected segment position

The firt valid segment position is start + offset.
Also add some more debug and a FIXME
segment: add method to offset the segment running-time

Add a method that can apply an offset to the calculated running-time of
a segment.
pad: use new segment offset method to apply the offset

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=721422
segment: take offset into account in _to_position()

Take the offset into account when converting between running-time and
segment positions.
tests: add unit test for segment _offset_running_time()

Add a unit test to check that positive and negative offsets are applied
correctly in various cases.
tests: improve check, also check stream-time
baseparse: do not ignore TIME segments

TIME segments are being ignored and a standard initialized
segment is used instead. This causes issues as not properly detecting
reverse playback or not cliping output based on the segment.

This seems to be a regression from one of the GstSegment/GstEvent
redesigns on the 0.10 -> 1.0 transition
gstobject: add FIXME and docs for the disabled notify on parent

We haven't found a way to re-enable emitting notify and deep-notify for parent
changes. Add a FIXME-2.0 and a doc blob on the property. See #693281.
basesrc: don't confuse GST_PAD_MODE_NONE and PULL

Use a switch-case to explicitly handle all pad-modes. This way we don't log an error when the pad is not yet activated.