qml: refactor qeinvoice.py

QEInvoice/QEInvoiceParser now properly split for mapping to Invoice type (QEInvoice)
and parsing/resolving of payment identifiers (QEInvoiceParser).
additionally, old, unused QEUserEnteredPayment was removed.

invoices are now never saved with user-entered amount if the original invoice
did not specify an amount (e.g. address-only, no-amount bip21 uri, or no-amount
lightning invoice). Furthermore, QEInvoice now adds an isSaved property so the
UI doesn't need to infer that from the existence of the invoice key.

Payments of lightning invoices are now triggered through QEInvoice.pay_lightning_invoice(),
using the internally kept Invoice instance. This replaces the old call path of
QEWallet.pay_lightning_invoice(invoice_key) which required the invoice to be saved
in the backend wallet before payment.

The LNURLpay flow arriving on InvoiceDialog implicitly triggered payment, this is
now indicated by InvoiceDialog.payImmediately property instead of inferrred from the
QEInvoiceParser isLnurlPay property.

electrum/gui/qml/components/InvoiceDialog.qml

@@ -11,7 +11,7 @@ ElDialog {
     id: dialog
 
     property Invoice invoice
-    property string invoice_key
+    property bool payImmediately: false
 
     signal doPay
     signal invoiceAmountChanged
@@ -392,13 +392,13 @@ ElDialog {
                 Layout.preferredWidth: 1
                 text: qsTr('Save')
                 icon.source: '../../icons/save.png'
-                enabled: invoice_key == '' && invoice.canSave
+                enabled: !invoice.isSaved && invoice.canSave
                 onClicked: {
-                    app.stack.push(Qt.resolvedUrl('Invoices.qml'))
                     if (invoice.amount.isEmpty) {
-                        invoice.amount = amountMax.checked ? MAX : Config.unitsToSats(amountBtc.text)
+                        invoice.amountOverride = amountMax.checked ? MAX : Config.unitsToSats(amountBtc.text)
                     }
                     invoice.save_invoice()
+                    app.stack.push(Qt.resolvedUrl('Invoices.qml'))
                     dialog.close()
                 }
             }
@@ -410,15 +410,10 @@ ElDialog {
                 enabled: invoice.invoiceType != Invoice.Invalid && invoice.canPay
                 onClicked: {
                     if (invoice.amount.isEmpty) {
-                        invoice.amount = amountMax.checked ? MAX : Config.unitsToSats(amountBtc.text)
-                        if (invoice_key != '') {
-                            // delete the existing invoice because this affects get_id()
-                            invoice.wallet.delete_invoice(invoice_key)
-                            invoice_key = ''
-                        }
+                        invoice.amountOverride = amountMax.checked ? MAX : Config.unitsToSats(amountBtc.text)
                     }
-                    if (invoice_key == '') {
-                        // save invoice if new or modified
+                    if (!invoice.isSaved) {
+                        // save invoice if newly parsed
                         invoice.save_invoice()
                     }
                     doPay() // only signal here
@@ -429,18 +424,14 @@ ElDialog {
     }
 
     Component.onCompleted: {
-        if (invoice_key != '') {
-            invoice.initFromKey(invoice_key)
-        }
         if (invoice.amount.isEmpty && !invoice.status == Invoice.Expired) {
             amountContainer.editmode = true
         } else if (invoice.amount.isMax) {
             amountMax.checked = true
         }
-        if (invoice.isLnurlPay) {
-            // we arrive from a lnurl-pay confirm dialog where the user already indicated the intent to pay.
+        if (payImmediately) {
             if (invoice.canPay) {
-                if (invoice_key == '') {
+                if (!invoice.isSaved) {
                     invoice.save_invoice()
                 }
                 doPay()