Move composer alias "?" icon to be inside field

Be more consistent with the user ID field and its copy button.

src/gui/Base/Buttons/FieldHelpButton.qml

@@ -0,0 +1,19 @@
+// Copyright Mirage authors & contributors <https://github.com/mirukana/mirage>
+// SPDX-License-Identifier: LGPL-3.0-or-later
+
+import QtQuick 2.12
+import QtQuick.Layouts 1.12
+import ".."
+
+HButton {
+    property string helpText
+
+    icon.name: "field-help"
+    iconItem.small: true
+    toolTip.text: helpText
+
+    onClicked: toolTip.instantShow()
+    onActiveFocusChanged: if (! activeFocus && toolTip.visible) toolTip.hide()
+
+    Layout.fillHeight: true
+}