Allow configuring patterns to loop

[cdauth]

When you press Play in the pattern placeholder or pattern editor, the player will loop the pattern if either one of the two conditions applies:

• The "Loop" option in the playback settings is enabled
• The pattern itself has loop: true set.

The problem is that the first one is not exported as part of shared links, and for the second one there is no UI. By default, when creating a new tune now, it creates a pattern “Tune” that has loop: true set, but there is no way for users to create an additional break that should be looped. We should add a setting for this to the pattern editor.