macos: Set activation policy in gst_macos_main and video sinks

Supersedes !4330 (closed)

The linked MR explains the issue thoroughly. Setting the policy to accessory by default ensures we can correctly re-activate windows after clicking away, Mission Control back to them, etc, as well as receive navigation events. In addition to the above MR, video sinks (glimagesink/osxvideosink) now set the policy to regular upon creating a window, so that an icon shows up in the dock - comes in handy when you lose a window during testing for example.

cc @lgeiger

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