Resize menu field if below minimum width in auto-size mode

... cancelling the normal item truncation behavior.
This funcationality comes from BeOS R5, we need to reproduce it for
backwards compat. KeymapSwitcher depends on it at least.

Minimum width is 20px, was set in last commit, comes from BeOS R5.

headers/private/interface/BMCPrivate.h

@@ -19,7 +19,6 @@
 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;