drivechain_client: send page groundwork

[arnemolland]

Adds the page shown under the "Send" tab; a form for txn details and fees.

This change also introduces a couple of components to sail_ui: DropdownButton and Checkbox -- I've made these for desktop use cases, but these are by no means final versions.

There's also some inconsistency in what style values are used and where; I've used the Material theme a fair bit here, but I'm not sure if I should replace them with sail_ui values (or introduce new values) as there's still some Material primitives in use.

Remaining:

• [x] Clipboard paste & clear field
• [ ] Address book
• [ ] Fee form
• [ ] Clear all
• [ ] Validation & RPC call

[octobocto]

I think we should avoid mixing material & sail_ui, and never use Theme.of(context).

Won't go completely without material, and when we must, override the colors from SailTheme.

[arnemolland]

I think we should avoid mixing material & sail_ui, and never use Theme.of(context).

Won't go completely without material, and when we must, override the colors from SailTheme.

I agree, just didn't know which one to go with here. I'll refactor and update!

[arnemolland]

I'll move some of the thrown together components to sail_ui, just needs some cleaning up. Was not sure if they would have place there or not given the Qt/native style I've gone for.

[octobocto]

I'll move some of the thrown together components to sail_ui, just needs some cleaning up. Was not sure if they would have place there or not given the Qt/native style I've gone for.

I'm thinking they can be chucked in there. It's fine with some inconsistencies in styling. We'll converge on a unified style later!

[arnemolland]

This is getting v big and I'd rather have multiple smaller, more reviewable ones. Setting this as ready for review and connecting the dots (form validation/submission/state stuff) in a follow-up.