win-dshow: Allow synchronous create/update

Allows the ability to (optionally) synchronously create/update a
directshow device source rather than always asynchronously update the
device.  This is useful if creating/destroying scenes/sources very
quickly, and helps minimize the risk of creating new directshow sources
that use the same device, yet may not activate because an existing
source may already exist.  To use, set "synchronous_activate" to true in
its settings when updating or creating.  Note that this setting will be
erased after it's used, and will not be saved to user settings, so it
must be set each time in order to be used.

Closes obsproject/obs-studio#1228