Add some functions to handle supplementary characters

The added functions are:

- wxUniChar::IsBMP()
- wxUniChar::IsSupplementary()
- wxUniChar::HighSurrogate()
- wxUniChar::LowSurrogate()

interface/wx/unichar.h

@@ -83,6 +83,82 @@ public:
      */
     bool GetAsChar(char *c) const;
 
+    /**
+        Returns true if the character is a BMP character (i.e.\ if its value is less than 0x10000).
+
+        @since 3.1.1
+    */
+    bool IsBMP() const;
+
+    /**
+        Returns true if the character is a BMP character (i.e.\ if its value is less than 0x10000).
+
+        @param value
+            The Unicode code point of the character.
+
+        @since 3.1.1
+    */
+    static bool IsBMP(wxUint32 value);
+
+    /**
+        Returns true if the character is a supplementary character (i.e.\ between 0x10000 and 0x10FFFF).
+
+        @since 3.1.1
+    */
+    bool IsSupplementary() const;
+
+    /**
+        Returns true if the character is a supplementary character (i.e.\ between 0x10000 and 0x10FFFF).
+
+        @param value
+            The Unicode code point of the character.
+
+        @since 3.1.1
+    */
+    static bool IsSupplementary(wxUint32 value);
+
+    /**
+        Returns the high surrogate code unit for the supplementary character.
+
+        @pre IsSupplementary() const
+
+        @since 3.1.1
+    */
+    wxUint16 HighSurrogate() const;
+
+    /**
+        Returns the high surrogate code unit for the supplementary character.
+
+        @param value
+            The Unicode code point of the character.
+
+        @pre IsSupplementary(wxUint32 value)
+
+        @since 3.1.1
+    */
+    static wxUint16 HighSurrogate(wxUint32 value);
+
+    /**
+        Returns the low surrogate code unit for the supplementary character.
+
+        @pre IsSupplementary() const
+
+        @since 3.1.1
+    */
+    wxUint16 LowSurrogate() const;
+
+    /**
+        Returns the low surrogate code unit for the supplementary character.
+
+        @param value
+            The Unicode code point of the character.
+
+        @pre IsSupplementary(wxUint32 value)
+
+        @since 3.1.1
+    */
+    static wxUint16 LowSurrogate(wxUint32 value);
+
     //@{
     /**
         Conversions to char and wchar_t types: all of those are needed to be