Add parachain

[evilrobot-01]

Adds support for connecting to parachains, with Statemint/e and the BridgeHub system parachains as examples.

Implements a number of the suggestions from #16:

• moves chain logic to models/chain.dart, so logic separate from presentation
• stores an async task in api.rs per chain, used for forwarding rpc responses to the corresponding StreamSink<String>
• stores and exposes the resulting stream as a broadcast stream within chain.dart/Chain
• uses provider dart package for pusing state change updates from chain.dart/Chain (e.g. as blocks received)
  • Stateful widgets converted to stateless as a result
• standardises on svg assets, removing the need for vec variants
• uses ExpansionTile for relay chain in app drawer
• adds ChainSyncStatus widget to show relay chain and parachain sync status when parachain selected

Closes #14

[evilrobot-01]

ExpansionTile switching is a little awkward for relay chains, might be better to remove the switch on expand header tap, especially as relay chain syncing status is now shown on parachain.

An alternative would be to simply have the network name in the expand header and have the relay chain as an option within, as with parachains. This aligns with polkadotjs/apps.

It also doesn't seem to maintain the expanded state, presumably due to the widget being rebuilt.

[stiiifff]

Amazing stuff @evilrobot-01, LGTM