Improve SVG import UX

Dropping SVG into an existing document canvas instead of importing it as a new layer/stack could display a context menu with options:
- Import as new layer
- Import as new Document

Dropping an SVG on an empty tab space should import it as new document:

Start by tracing the SVG drop handling for the existing document canvas and the empty tab area. Check how import targets are selected and how context menus are presented; done means canvas drops offer layer or document import, while empty-tab drops create a new document.

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