[Cashtab] Show liquid XEC and XECX amounts under combined balance

Summary: Improve visibility of XEC + XECX split. User can tap card to toggle between combined and split balances. Also tooltip on hover (web and extension).

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D20346