Add ability to lookup Custom Token prices on DEX

Description:
Add a feature for each token to add a DEX where the price can be found or automatically search the most popular DEX for token prices.

Purpose:
So that each trading token in your wallet will show a current value. For those of us participating in a lot of presale/ICO, most of my tokens do not show a value.

Extension/Mobile/Both:
Extension. I don’t use mobile with my Ledger.

Images/Attacments: