Source metadata editing panel

Counterpart of #36, where the same metadata can be edited in a form instead of being displayed in a static listing.

## Depends on

#11. Probably wise to implement after #36. Should use multiple instances of #53 as subviews, either directly or through (an intermediate subview shared with) #54.

## Constructor options

name | description
---|---
`model` | Souce of which the metadata will be edited.

## Rendering

Like #36, but as an editable form. Save/reset buttons instead of the edit button. Close button is required.

## Special methods

name | description
---|---
`prefill(metadata)` | (optional) Prefill some data in the form which differ from the `model`'s current metadata and return `this`.

## Events

name | description
---|---
`close` (view) | Triggered when the close button is clicked.

## Notes

Like in #34, this view shouldn't be responsible for anything that happends outside of the panel.