qml: ElTextArea: hide placeholder text on user input

When the user enters something into the ElTextArea the placeholder
text should be hidden so it doesn't conflict with the user input.
This affects e.g. the lnurl pay dialog.

electrum/gui/qml/components/controls/ElTextArea.qml

@@ -17,7 +17,7 @@ Flickable {
     property alias background: rootpane.background
     property alias font: edit.font
     property alias inputMethodHints: edit.inputMethodHints
-    property alias placeholderText: edit.placeholderText
+    property string placeholderText
     property alias color: edit.color
     property alias topPadding: rootpane.topPadding
     readonly property bool anyActiveFocus: activeFocus || edit.activeFocus
@@ -55,6 +55,7 @@ Flickable {
             width: parent.width
             focus: true
             wrapMode: TextEdit.Wrap
+            placeholderText: edit.text.length ? "" : root.placeholderText
             onCursorRectangleChanged: root.ensureVisible(cursorRectangle)
             onTextChanged: root.textChanged()
             background: Rectangle {