Minor changes.
Qt5 signal/slot style.
minor
Qt5 style signal/slot
Introduce timeline2
Cleanup
1. Qt5 style signal-slot
1. Simplify playback event.
1. Simplify jump to start/end event process.
1. Simplify the loop button event.
Minor cleanup.
[Fix] timeline didn't update on playing
[Fix] timeline did not repaint immediately after current layer changed.
Fix merge conflict