[electrum] don't open a blocking popup when an invalid URI is pasted in the Pay To field

Summary:
Use a temporary (5 seconds) non-blocking popup label instead, the same one used to signal that an update is available or that the CashFusion server is not responding
This allows the user to keep editing the URI in the dialog without having to close a blocking popup dialog at each keystroke.

Depends on D15384

Test Plan:
Paste a bad URI, check that there is an more discrete error popup (in the cases that previously raised a blocking popup dialog) and that you can keep editing the URI.

Test URIs:

• duplicate parameter: ecash:qrh3ethkfms79tlcw7m736t38hp9kg5f7gycxeymme?amount=40.00&label=test&amount=30.00
• ignored parameter: ecash:qrh3ethkfms79tlcw7m736t38hp9kg5f7gycxeymme?spam=foo
• Bad parameter:
  • incorrect checksum in address: ecash:qrh3ethkfms79tlcw7m736t38hp9kg5f7gycxeymma
  • non-numeric amount: `ecash:qrh3ethkfms79tlcw7m736t38hp9kg5f7gycxeymme?amount=deadbeef
  • non-hex op_retrun_raw: ecash:qrh3ethkfms79tlcw7m736t38hp9kg5f7gycxeymme?op_return_raw=deadbeefzz

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15385