Import dialog

[achow101]

Right now users have to use the RPC console to do imports. This is not a good experience. We should have a dialog that lets people import things into a wallet. This should probably mirror importmulti and importdescriptors; we can probably reuse some of that code.

[Sjors]

Concept ACK

This might be an alternative approach to the unified wallet creation & import step in #4 (or at least a way to make that PR slightly smaller if it's a problem).