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/toolpanel.cc

@@ -61,7 +61,7 @@ FoldableToolPanel::FoldableToolPanel(Gtk::Box* content, Glib::ustring toolName,
         label->set_alignment(Gtk::ALIGN_START, Gtk::ALIGN_CENTER);
         titleHBox->pack_start(*label, Gtk::PACK_EXPAND_WIDGET, 0);
 
-        RTImage *image = Gtk::manage (new RTImage("one-to-one-small.png"));
+        RTImage *image = Gtk::manage (new RTImage("one-to-one-small"));
         image->set_tooltip_text(M("TP_GENERAL_11SCALE_TOOLTIP"));
         titleHBox->pack_end(*image, Gtk::PACK_SHRINK, 0);