Add a wallet selector and add support for Talisman wallet.

• Problem
  When trying to connect the Talisman wallet, it's not officially supported and while it DOES work (you can connect your wallet and sign TXs), it's not the smoothest experience.

• Solution
  Talisman has created an easy library for Darwinia Apps to implement (See below). If you want to use your own UI, you can use just part of the package (to do most of the hardwork for you!) and add your own UI on top. Or you can use our built in UI to make the process easier.
  https://github.com/TalismanSociety/talisman-connect

As more wallets come to the dotsama ecosystem, Apps will need a better way to manage them, this feature request provides some future proofing and also let's your Apps support Talisman!
You can see a working implementation of this at -> https://app.talisman.xyz/

• User journey

As a user of your Apps
Given I click, Connect
Then I want to see a modal display that allows me to select my preferred wallet.

First time user
When I select Talisman
Then I want to see a dialog appear showing the different wallets I could link (this is handled by the extension)
And after selecting one or more wallets, the modal should disappear and the Apps should re-load with my data

Returning user
Then when I click the wallet of choice, and select Talisman
And after selecting one or more wallets, the modal should disappear and the Apps should re-load with my data.