Customize the menu to fit our style (#2535)

* Style and align with a rounded border for combobox popups.

* Convert this menu to use the new style as well.

Signed-off-by: Adam Treat <treat.adam@gmail.com>

gpt4all-chat/qml/MyMenuItem.qml

@@ -0,0 +1,20 @@
+import QtCore
+import QtQuick
+import QtQuick.Controls
+import QtQuick.Controls.Basic
+
+MenuItem {
+    id: item
+    background: Rectangle {
+        color: item.highlighted ? theme.lightContrast : theme.darkContrast
+    }
+
+    contentItem: Text {
+        leftPadding: 10
+        rightPadding: 10
+        padding: 5
+        text: item.text
+        color: theme.textColor
+        font.pixelSize: theme.fontSizeLarge
+    }
+}