Full BIP44 address hierarchy in wallet GUI

**Feature description**

BIP44 address hierarchy in wallet GUI

**Is your feature request related to a problem? Please describe.**
Display multiple accounts per wallet .

**Describe the solution you'd like**
Two-level hierarchy in wallet GUI . Top level for wallets . Next level for BIP44 `account'` index node.

**Describe alternatives you've considered**
Starting the account hierarchy at BIP44 `account` level has multiple drawbacks. Firstly all accounts would be siblings hence that relation to a mnemonic is non-trivial , not to say non-existent. Besides the .

**Additional context**
This feature not needed in v1 since SkyFiber plugin only supports `account=0` but other cryptocurrencies do actually implement it. Changes in backend should be ready once #177 be finished .

BIP44 `cointype` should be represented by other GUI controls . See #178

**Possible implementation**

- [ ] Two-level hierarchy mockups
- [ ] Implement GUI hierarchy considering changes in #177
- [ ] Implement CRUD actions for individual BIP44 `account'` nodes