Dropdown menu for new accounts and separate import/binding abilites

[Tbaut]

After a brainstorming we agreed on having 1 view per type of import. It'd look much simpler for users spliting the current component in 3 different components.

from https://github.com/paritytech/fether/issues/264

The wording for Signer account should be "Bind a Signer account" to differentiate from "importing".