Improve SVG import UX

[j-jasz]

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:

[kanishk6103]

Hey! I am working on this one. @j-jasz @Keavon