[Feature] Advanced feature allowing sending user (manual Send) to select delivered currency

[WietseWind]

Idea makes sense. Not for all users, but for advanced sending.

So:

1. User "Send"
2. Enters currency & amount
3. Now (under Currency section) we can check the destination to see which Trust Lines are setup, and check for DEX liquidity. We can then show an Advanced button, and allow someone to select the currency to DELIVER.

Right now: XRP » XRP, and IOU » IOU (if trust line) or IOU » XRP (dex conversion) if the recipient doesn't have the trust line.

With this change (addition) one can say:

I want to send XRP and deliver EUR
I want to send EUR and deliver USD

Instead of us just allowing (which will still be default behavious):

XRP » XRP
IOU » XRP
IOU » Same IOU

@thisistriss Adding you for possible styling considerations.

[WietseWind]

Should play nice with #109 too, maybe the Advanced selection should (could) be at the location where the notification is displayed (in the currency)

[canny]

This issue has been linked to a Canny post: P2P remittance leveraging on the DEX 🎉

[WietseWind]

Duplicates:
#129
#76
#108

Will be implemented as xApp (anything to anything)