- move "open property browser" back to SPACE (for now any modifier combination

other than Shift-SPACE or SPACE alone). The move to Shift-Enter was bad,
  as all Return combinations are filtered by open dialogs with <default>
  set. SPACE remains reserved for communication functions (PTT), though, and
  the property browser will be moved away if more combinations are needed for it.
- some comments in the file head

keyboard.xml

@@ -6,10 +6,13 @@ The *-key in fg/osg/osgviewer is handled in the C++ code.
 
 Regular keycodes go up to 255; special keys start at 256, and can be
 calculated by adding 256 to the GLUT key value in glut.h.
+
+All Nasal code in this file lives in a common Nasal namespace __kbd.
+All <nasal> blocks on the top level are executed in the order from
+top down before the key bindings are parsed.
 -->
 
 <PropertyList>
-
  <nasal>
   <script>
       var shift = props.globals.getNode("/devices/status/keyboard/shift");
@@ -24,13 +27,13 @@ calculated by adding 256 to the GLUT key value in glut.h.
       var space = func(state, mod) {
           if (!state) {
               space_release();
-              return space_release = func {}
+              return space_release = func {};
           }
           if (mod == 0 or mod == 1) {
               controls.ptt(mod + 1);
               space_release = func { controls.ptt(0) }
           } else {
-              gui.popupTip("SPACE " ~ mod);
+              gui.property_browser();
           }
       }
   </script>
@@ -120,12 +123,6 @@ calculated by adding 256 to the GLUT key value in glut.h.
    <property>/controls/flight/rudder</property>
    <step type="double">0.05</step>
   </binding>
-  <mod-shift>
-   <binding>
-    <command>nasal</command>
-    <script>gui.property_browser()</script>
-   </binding>
-  </mod-shift>
  </key>
 
  <key n="14">