Theme tweaks in light of new icons, #4469

- Button and combobox sizes consistent.
- Padding around left and right edges of comboboxes.
- Fixed bug in green channel preview togglebutton icon always being
  shown as "off".
- Added CSS classes to various elements.

rtgui/distortion.cc

@@ -29,6 +29,7 @@ Distortion::Distortion (): FoldableToolPanel(this, "distortion", M("TP_DISTORTIO
     rlistener = nullptr;
     autoDistor = Gtk::manage (new Gtk::Button (M("GENERAL_AUTO")));
     autoDistor->set_image (*Gtk::manage (new RTImage ("distortion-auto-small.png")));
+    autoDistor->get_style_context()->add_class("independent");
     autoDistor->set_alignment(0.5f, 0.5f);
     autoDistor->set_tooltip_text (M("TP_DISTORTION_AUTO_TIP"));
     idConn = autoDistor->signal_pressed().connect( sigc::mem_fun(*this, &Distortion::idPressed) );