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/crop.cc

@@ -128,6 +128,7 @@ Crop::Crop():
     pack_start (*hb2, Gtk::PACK_SHRINK, 4);
 
     selectCrop = Gtk::manage (new Gtk::Button (M("TP_CROP_SELECTCROP")));
+    selectCrop->get_style_context()->add_class("independent");
     selectCrop->set_image (*Gtk::manage (new RTImage ("crop-small.png")));
 
     pack_start (*selectCrop, Gtk::PACK_SHRINK, 2);