Separate Snapping Toggles for Video Sequencer and Preview

Currently, the snapping toggle of the video sequencer is shared between the Preview and Sequencer View types. This is problematic, as the purpose of the snap tool in the Preview is to snap the selected strip into predetermined positions on the screen. But the purpose of snap tool in the Sequencer is to snap strips to different times.

I propse separating these tools from each other and removing the shared behavior.