Implement wxUILocale::CompareStrings() for Unix systems

This required changing CompareStrings() to be a method of wxUILocale
object, rather than just as a static function, as we must only allocate
the locale_t object once, and not during each to this function, as this
could make it unusably slow when using it as a comparison function when
sorting a large list of strings.

This is also more efficient under Mac, where we can similarly allocate
NSLocale only once and even marginally more efficient under MSW, where
we don't have to construct the locale string during each call. And,
under all platforms, it also simplifies code by separating this function
implementation from the initialization of wxUILocaleImpl.

Also document that case-insensitive comparison is not available under
Unix and adjust the tests accordingly.

interface/wx/uilocale.h

@@ -17,7 +17,12 @@ enum
     /// Compare strings case-sensitively, this is the default.
     wxCompare_CaseSensitive   = 0,
 
-    /// Ignore strings case when comparing.
+    /**
+        Ignore strings case when comparing.
+
+        Note that this flag is not supported under POSIX systems, where it is
+        simply ignored.
+     */
     wxCompare_CaseInsensitive = 1
 };
 
@@ -99,10 +104,9 @@ public:
 
         In the simplest case, this can be used as following:
         @code
-            const wxUILocale loc("fr");
+            const wxUILocale loc(wxLocaleIdent("fr"));
         @endcode
-        but more precise locale identifiers can be used, see wxLocaleIdent
-        description for more details.
+        see wxLocaleIdent description for more details.
 
         If @a localeId is not recognized or not supported, default ("C") locale
         is used instead.
@@ -110,26 +114,24 @@ public:
     explicit wxUILocale(const wxLocaleIdent& localeId);
 
     /**
-        Compares two strings using comparison rules of the given locale.
+        Compares two strings using comparison rules of this locale.
 
         @param lhs
             First comparing string.
         @param rhs
             Second comparing string.
-        @param localeId
-            Represents platform dependent language name.
-            @see wxLocaleIdent for details.
         @param flags
             Can be used to specify whether to compare strings case-sensitively
-            (default) or not, by specifying ::wxCompare_CaseInsensitive.
+            (default) or not, by specifying ::wxCompare_CaseInsensitive (note
+            that this flag only works under MSW and Mac and is simply ignored
+            under the other platforms).
         @return
             -1 if lhs less than rhs.
             0 if lhs equal to rhs.
             1 if lhs greater than rhs.
      */
-    static int CompareStrings(const wxString& lhs, const wxString& rhs,
-                              const wxLocaleIdent& localeId = wxLocaleIdent(),
-                              int flags = wxCompare_CaseSensitive);
+    int CompareStrings(const wxString& lhs, const wxString& rhs,
+                       int flags = wxCompare_CaseSensitive) const;
 
     /**
         Get the platform-dependent name of the current locale.