Cached 'has' result in initialize()

Also amended a bit of README that was missed and added a memory usage in the RootView for widgets here memory is limited, to serve as a warning to users who are likely to complain.

source/Globals.mc

@@ -33,4 +33,6 @@ class Globals {
     // Needs to be long enough to enable a "double ESC" to quit the application from
     // an ErrorView.
     static const scApiResume    = 200;  // ms
+    // Warn the user after fetching the menu if their watch is low on memory before the device crashes.
+    static const scLowMem       = 0.95; // percent as a fraction.
 }