GUI control to choose transaction signer

**Feature description**

User interface should allow for choosing transaction signer strategy

**Is your feature request related to a problem? Please describe.**
Multiple signers are supported in the backend, yet frontend does not allow for signing transactions .

**Describe the solution you'd like**
Should multiple signers be available for a specif wallet , a drop down button (or combobox) should allow for choosing which one to use for signing a given transaction . The button should display the name of chosen strategy. If name is too long , use ellipsis to trim it .

An option (i.e. switch UI control) should be added for each wallet so as to choose one of two possible signing modes

- **Sign all with** (default) : All inputs originating from wallet are signed with chosen strategy. Every input corresponding to addresses related to a wallet should be signed by same signer.
- **Advanced** : User should be able to configure signer for individual inputs

Default signer value should be the input's wallet itself.

**Describe alternatives you've considered**
Radio buttons occupy a lot of space.

**Additional context**
Backend upgraded for this in #182 for #138 .

**Possible implementation**

- [ ] Implement QT signer selector control
- [ ] Add signing mode switch , only if wallets involved in transaction signing support multiple srategies
- [ ] Group inputs in GUI by wallet
- [ ] Detect whether wallet supports multiple signers
- [ ] Add signer selector controls in transaction input GUI row when mode set to `advanced`
- [ ] Add a single signer selector per wallet when signing mode set to `simple`
- [ ] Connect with backend and sign txn