Fix timecode shift when reopening a detx file

Dear Joker developers,

When opening a detx doc for a second time, after it was opened once and the video (24 fps) was opened too, the timecode is wrong, shifted by 2'30.

The problem boils down to conflicting settings for the timecode type of the strip clock. In JokerWindow::openDocument it is set to the doc timecode type, and in Synchronizer::onVideoTCTypeChanged it is set to the video engine timecode type.

This commit fixes that by initializing the strip clock timecode type to the one of the video engine in JokerWindow::openDocument.

(Please note that using a 24 fps video presents other issues fixed in PR#214.)

Thanks !