GasPrice selection for contract execution

[jacogr]

Closes https://github.com/ethcore/parity/issues/3510

https://youtu.be/zT0ioLLxuA0

(Modal states should really be converted to MobX stores like @ngotchac has done with Transfer, much cleaner to maintain)

[ngotchac]

It looks good, but still misses some places for gas price selection (at least contract deployment). As discussed, I would rather have this available (as well) in the Signer view/overlay.

As a side note, I think modals should be refactor, as it shouldn't be that compilcated to manage modal step, create new ones, etc... But this is just a side note ;)

[jacogr]

Signer is currently WIP as per https://github.com/ethcore/parity/issues/3016 & https://github.com/ethcore/parity/issues/2605

First step was getting the component out, second to make sure it works as expected in another use (here), then signer - which has got additional complexities.