Add a reactive variable returning a list of transactions

[davidyuk]

Basically, to extract it from Base aepp https://github.com/aeternity/aepp-base/blob/e836ca6ae3440b31e661964ebf966eff5bd669d7/src/store/plugins/ui/observables.js#L207-L310 It should return transactions from middleware with the ability to subscribe for new transactions, load the next page of transactions.

This would be useful for aepps and wallets.

Preferably, it shouldn't depend on a specific reactivity engine but be somehow plugable.