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/lensgeom.cc

@@ -31,6 +31,7 @@ LensGeometry::LensGeometry () : FoldableToolPanel(this, "lensgeom", M("TP_LENSGE
 
     autoCrop = Gtk::manage (new Gtk::Button (M("TP_LENSGEOM_AUTOCROP")));
     autoCrop->set_image (*Gtk::manage (new RTImage ("crop-auto-small.png")));
+    autoCrop->get_style_context()->add_class("independent");
     pack_start (*autoCrop, Gtk::PACK_SHRINK, 2);
 
     packBox = Gtk::manage (new ToolParamBlock ());