Initial commit for real hidpi support

Note: This commit has only been tested on MacOS

Changes:
- Icons now use the native hidpi support from Gtk (through Icon Theme)
- Icons are now directly generated from scalable file (i.e. SVG file)
- Widget sizes are scaled based on DPI and scale factor
- Font size is scaled based on DPI and scale factor

rtgui/thresholdadjuster.cc

@@ -99,7 +99,7 @@ void ThresholdAdjuster::initObject (Glib::ustring label, bool editedcb)
     hbox->pack_start (*this->label);
 
     reset = Gtk::manage (new Gtk::Button ());
-    reset->add (*Gtk::manage (new RTImage ("undo-small.png", "redo-small.png")));
+    reset->add (*Gtk::manage (new RTImage ("undo-small", Gtk::ICON_SIZE_BUTTON)));
     reset->set_relief (Gtk::RELIEF_NONE);
     reset->set_tooltip_markup (M("ADJUSTER_RESET_TO_DEFAULT"));