add progress bar when a screen share started

# Conflicts:
#	clients/flash/air-client/src/org/bigbluebutton/air/common/views/IOSVideoView.as

Add BBBRtmpPlayer ANE

 - toggle presentation downloads

Fix handling webcam videos that end early in recording processing

In some cases (due to network issues), the webcam video can be shorter
than the time between the start/stop events. Pad the input video with a
blank video to make sure that there's input to the video tiling filters,
to fix a problem where the video won't render correctly with ffmpeg v3.4
and later.

[AIR] Stop the active poll when the result is shown

Implement new polling popup

# Conflicts:
#	clients/flash/air-client/src/org/bigbluebutton/air/main/views/MenuButtons.as

Use PNG slides for iOS

Cleanup AIR settings

 - generate png files for uploaded presentation