[UI improvement] Destination account & destination tag flow

[WietseWind]

Right now, when sending XRP, first the source account, currency and amount are entered.
After that, the destination is selected (enter account / scan a QR).
Then, one goes to the final screen to enter meta info and ... The destination tag.

That doesn't make sense; the Destination tag is part of the destination so it belongs on the screen where the destination account is entered and selected. It can even be very confusing to end users as they can't find a place to enter the tag when selecting the destination.

Here's what we should do (imo) (screenshots by @thisistriss from the original designs).

• if user originates from pasting/scanning an r-address / x-address WITH destination tag:
  1. Select address
  2. Skip (mockup) destination tag entry popup
  3. Show the tx summary (but with tag DISPLAYED) in the "To" section, with a "edit" icon, triggering the (mockup) destination tag entry popup
• if user originates from pasting/scanning an r-address WITHOUT destination tag:
  1. Select address
  2. SHOW (mockup) destination tag entry popup, if tag for the account is mandatory: require a tag (next disabled if not entered) and show the "requires a ..." text. If the tag for the account is NOT mandatory, allow entry but also allow NEXT without tag.
  3. Show the tx summary (but with tag DISPLAYED) in the "To" section, with a "edit" icon, triggering the (mockup) destination tag entry popup. Show "No destination tag" with edit icon if none is entered.

[WietseWind]

Involves:

• [Improvement] Tag warning if no dflag set but most incoming txs have dtags #125
• [Improvement] Respect amount from XrplDestination (QR) #124
• [Improvement] Add header (X-XummNet) to handle-lookup endpoint #122
• [Improvement] Add PayId source in search result screen  #121
  ...

[WietseWind]

^^ States:

1. No tag required (so not asked with overlay after NEXT on destination selection): Button: [Add destination tag]
2. Destination tag already entered: Show tag + [ Edit Destination tag]

[WietseWind]

Note

When scanning a QR code with a destination address, and possibly tag: DIRECTLY open the tag panel, as selecting the result is not required anyway since there will only be one address in the scanned QR. If the tag is in the scanned QR as well: prefilled, but still show the tag panel.