qml: Validate wallet name during wallet creation

[pseudoramdom]

Fixes #571

When creating a wallet, the name is checked for

• invalid/empty names
• discovered wallet names in wallet dir
• existing loaded wallets

The same checks are performed for external-signer wallets.

Review notes

This PR is stacked on top of #547. Now rebased on top of qt6

Screenshots

[pseudoramdom]

I could've written unit test, but the scaffolding for test_walletcontroller is all in #551 already.
Felt it's easier to add later rather than picking that commit and dropping it later when #551 is merged. Thoughts @johnny9 ?

[johnny9]

I could've written unit test, but the scaffolding for test_walletcontroller is all in #551 already.
Felt it's easier to add later rather than picking that commit and dropping it later when #551 is merged. Thoughts @johnny9 ?

I'm fine with that.

[pseudoramdom]

558cf9c
Rebased on top of qt6 now