feat(ui): grey out proxy settings if no proxy selected

This is a feature proposed in #2760.
Disabled port (QSpinBox) and address (QLineEdit) are greyed out if no proxy is selected.

ui/settings/mainContent.css

@@ -39,6 +39,11 @@ QSpinBox
     background: white;
 }
 
+QSpinBox:disabled
+{
+    background: lightGrey;
+}
+
 QPushButton
 {
     background: white;
@@ -66,6 +71,11 @@ QLineEdit
     background-color: white;
 }
 
+QLineEdit:disabled
+{
+    background-color: lightGrey;
+}
+
 QTabWidget
 {
     background-color: white;