app: make error-console highlighting criterion configurable

Currently, the error console is highlighted (shown/blinked) only
upon errors; however, warnings, which are not shown on the
statusbar while the error console is open, often also contain
important information.

Allow the user to configure which message types (errors, warnings,
and regular messages) highlight the error console, using a new
"highlight" submenu in the error-console menu.  Add corresponding
config options, saved in sessionrc.  By default, highlight the
error console unpon both errors and warnings.

menus/error-console-menu.xml

@@ -8,5 +8,11 @@
     <separator />
     <menuitem action="error-console-save-all" />
     <menuitem action="error-console-save-selection" />
+    <separator />
+    <menu action="error-console-highlight">
+      <menuitem action="error-console-highlight-error" />
+      <menuitem action="error-console-highlight-warning" />
+      <menuitem action="error-console-highlight-info" />
+    </menu>
   </popup>
 </ui>