[gui] PSBT Operations Dialog (sign & broadcast)

Add a "PSBT Operations" dialog, reached from the "Load PSBT..." menu
item, giving options to sign or broadcast the loaded PSBT as
appropriate, as well as copying the result to the clipboard or saving
it to a file.

src/psbt.cpp

@@ -214,6 +214,17 @@ bool PSBTInputSigned(const PSBTInput& input)
     return !input.final_script_sig.empty() || !input.final_script_witness.IsNull();
 }
 
+size_t CountPSBTUnsignedInputs(const PartiallySignedTransaction& psbt) {
+    size_t count = 0;
+    for (const auto& input : psbt.inputs) {
+        if (!PSBTInputSigned(input)) {
+            count++;
+        }
+    }
+
+    return count;
+}
+
 void UpdatePSBTOutput(const SigningProvider& provider, PartiallySignedTransaction& psbt, int index)
 {
     const CTxOut& out = psbt.tx->vout.at(index);