poll: Prevent false-negative from WAKE_EVENT() on W32

SetEvent() seems to not call SetLastError(0) internally, so checking last
error after calling SetEvent() may return the error from an earlier W32 API
call. Fix this by calling SetlastError(0) explicitly.

Currently WAKE_EVENT() code is cramped into a macro and doesn't look to be
entirely correct. Particularly, it does not check the return value of
SetEvent(), only the thread-local W32 error value. It is likely that SetEvent()
actually just returns non-zero value, but the code mistakenly thinks that the
call has failed, because GetLastError() seems to indicate so.

https://bugzilla.gnome.org/show_bug.cgi?id=733805
multiqueue: Post errors ourselves if they are received after EOS

After EOS there will be no further buffer which could propagate the
error upstream, so nothing is going to post an error message and
the pipeline just idles around.
queue: Post errors when receiving EOS after downstream returned an error

There might be no further data flow that would allow us to propagate the
error upstream, causing nobody to post an error at all.
queue2: Post errors if we receive EOS after downstream reported an error

There will be no further data flow that would allow us to propagate the
error upstream, causing nobody at all to post an error message.
element: Clarify docs about gst_element_get_request_pad() and remove deprecation part

This function is not really pad or slow for the common case of requesting a
pad with the name of the template. It is only slower if you to name your pads
directly instead of letting the element handle it.

Also there's no reason to deprecate it in favor of a more complicated function
for the common case.
bin: gst_bin_new() can accept NULL as name
bin: Use allow-none instead of nullable until we depend on a new enough GI version
multiqueue: Only handle flow returns < EOS as errors, not e.g. flushing
base: remove g-i annotation that makes older g-ir-scanner crash

Just remove one skip annotation that causes this:

  ** (g-ir-compiler:12458): ERROR **: Caught NULL node, parent=empty

with older g-i versions such as 1.32.1.
base: and fix build with new g-i again