qt: ensure translator comments end in full stop

This ensures that all gui translator comments end in a full stop.
If a comment does not end in a full stop, a translator may think 
that the rest of the comment is being cut off.

While here, add a colon to the word "see" for any comments
touched which point to look at a link.

src/qt/sendcoinsdialog.cpp

@@ -200,7 +200,7 @@ void SendCoinsDialog::setModel(WalletModel *_model)
         ui->optInRBF->setCheckState(Qt::Checked);
 
         if (model->wallet().hasExternalSigner()) {
-            //: "device" usually means a hardware wallet
+            //: "device" usually means a hardware wallet.
             ui->sendButton->setText(tr("Sign on device"));
             if (gArgs.GetArg("-signer", "") != "") {
                 ui->sendButton->setEnabled(true);