PixelPaint: Make tools use the new API for property change shortcuts

Every tool that has a slider now registers the primary/secondary
sliders and now uses the same keyboard shortcuts to modify the
primary and secondary properties. `[` and `]` for the primary,
`{` and `}` for the secondary.

Userland/Applications/PixelPaint/GuideTool.cpp

@@ -206,6 +206,7 @@ GUI::Widget* GuideTool::get_properties_widget()
         snapping_slider.on_change = [&](int value) {
             m_snap_size = value;
         };
+        set_primary_slider(&snapping_slider);
     }
 
     return m_properties_widget.ptr();