Allow calling wxStrchr() with a narrow string and wide character.

Calls to wxStrchr(char-string, wide-char) would previously fail if wide
character couldn't be converted to a single character in the current locale
encoding. Change it to simply return NULL in this case as it's a safe and
useful generalization: a narrow string will never contain a wide character not
representable in the current locale.

Add wxUniChar::GetAsChar() to help with implementing this.

Closes #11487.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62738 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/unichar.h

@@ -58,6 +58,30 @@ public:
     */
     bool IsAscii() const;
 
+    /**
+        Returns true if the character is representable as a single byte in the
+        current locale encoding.
+
+        This function only returns true if the character can be converted in
+        exactly one byte, e.g. it only returns true for 7 bit ASCII characters
+        when the encoding used is UTF-8.
+
+        It is mostly useful to test if the character can be passed to functions
+        taking a char and is used by wxWidgets itself for this purpose.
+
+        @param c
+            An output pointer to the value of this Unicode character as a @c
+            char. Must be non-@NULL.
+        @return
+            @true if the object is an 8 bit char and @a c was filled with its
+            value as char or @false otherwise (@a c won't be modified then).
+
+        @see IsAscii()
+
+        @since 2.9.1
+     */
+    bool GetAsChar(char *c) const;
+
     //@{
     /**
         Conversions to char and wchar_t types: all of those are needed to be