QRCode : add a method linphone_core_enable_persistent_qrcode_video_preview in API to enable the QRCode scanning for multiple scanning. Clarify the docstring of the method linphone_core_enable_qrcode_video_preview that scans only one QRCode. Add a test.
Note 1: to enable the scan of several QRCodes, the method linphone_core_enable_persistent_qrcode_video_preview or the method linphone_core_video_preview_enabled has to be called before the start of the video, in order to create the QRCode decoder. Then during the video the method linphone_core_enable_persistent_qrcode_video_preview can be used to enable and disable the continuous QRCode scanning several times.
Note 2: when the persistent QRCode scanning is enabled, a QRCode can be detected several times, as the detection is reset immediately, then several notifications are sent for the same QRCode.