Revert "ghostpad: copy sticky events to SRC ghostpads"

This reverts commit 8162a583a4dd68582bf186e2e47a8f0d68fa1980.

Automatically copying the sticky events makes it impossible for apps
and elements to filter the events with event probes. This causes
regressions (See #719437). The best option is to let the app/element
copy and filter the events themselves after the ghostpad target is
set.
inputselector: handle gap events

Use gap events to advance the selector's pad position.

This is relevant to keep sync_streams mode working when one of the
streams doesn't have data all the time.
gstpad: drop assertion on gst_pad_peer_query_position

It is a 'both' query, so it can be sent both ways
multiqueue: Wake up on reconfigure event

After patch bda406c4, the state of the singlequeue was set to OK, but nothing
would then wake up the thread, as the other wakeup functions only look at
singlequeues that are marked as having received as not-linked.

https://bugzilla.gnome.org/show_bug.cgi?id=708200
tools: gst-launch: don't try to remove already-removed GSource from main loop

It's considered a programming error in recent GLib versions now.
We may already have removed the source by returning FALSE from
the callback if it was fired. Fixes warning with newer GLibs
when interrupting a pipeline with Control-C.
tests: fix GstBus unit test with latest GLib

g_source_remove() works on the default main context, and
we're doing things with a custom context. Fixes warning
with newer GLib versions.