Returning promise in $.wikibase.listview.enterNewItem()

Improves code structure and removes need to register event listeners in most cases.

Change-Id: Ic99a1f44d991f4d8881b11686fed523ffcda0435
Focus referenceview when adding a new referenceview

Bug: T76860
Change-Id: I3b3b677378170a7f3d7a643cb0133701aaeadd1a
Focus snakview when adding a new snakview to a referenceview

Bug: T76860
Change-Id: I880efbb0f6e133ffa4dac36124fb1a3d522329fe
Merge "Returning promise in $.wikibase.listview.enterNewItem()" into wmf/1.25wmf12c
Merge "Focus referenceview when adding a new referenceview" into wmf/1.25wmf12c
Merge "Focus snakview when adding a new snakview to a referenceview" into wmf/1.25wmf12c