Move constants to BMCPrivate.h and refactor

We use these constants in both MenuField.cpp and BMCPrivate.cpp

Incorporate kMarginWidth into kPopUpIndicatorWidth.

A small code simplication in FrameResized() along with replacing bare numbers
with magic constants.

headers/private/interface/BMCPrivate.h

@@ -16,6 +16,12 @@
 #include <MessageFilter.h>
 
 
+static const float kVMargin = 2.0f;
+static const float kMinMenuBarWidth = 20.0f;
+	// found by experimenting on BeOS R5
+static const float kPopUpIndicatorWidth = 13.0f;
+
+
 class BMessageRunner;