Improve stacking UX

[philiphacks]

Is your feature request related to a problem? Please describe.
When users are stacking with a stacking pool, they delegate STX to that pool. The STX tokens remain unlocked in the user's wallet until the pool actually stacks (& commits) them for the next cycle. When they use other apps later (in between delegation and a new stacking cycle), these STX are still unlocked in their wallet. This becomes cumbersome to remember and is confusing for users: they can still spend the STX, but often they forget about the delegated STX. With an app like Stacking DAO, we now take into account delegated but not yet locked STX and subtract that from the STX balance, but that creates a discrepancy between what the wallets show and the Stacking DAO UI. This also creates confusion. Overall the UX is degraded due to those 3 states: STX can be in the wallet (normal balance), delegated but unlocked or locked. This is one state too many (imho). Also it is possible to delegate more STX than the wallet balance but I'm not sure which use case this enables (it turns out not to be a useful feature in practice).

Describe the solution you'd like
Ideally the delegated STX move to a separate account (owned by the same wallet address) that "freezes" the delegated STX until it can be locked by the pool. This way no extra logic is needed by apps, and two states exist for the STX: unstacked in the wallet or stacked in the separate account. This is also how Solana operates: once you stake, the stake moves to a separate (sub-)account owned by the address.