Fix truncating CJK mnemonics in control labels

13068d3603 introduced code for stripping
CJK mnemonics (i.e. trailing " (&X)") from the menu items, but due to
the use of wxStripMenuCodes() in wxControl::GetLabelText(), it also
applied to the control labels, resulting in wrongly measuring their size
(because the text used for measurement didn't include the "(&X)" part)
and truncating them in display.

Fix this by adding an explicit wxStrip_CJKMnemonics and using it only in
the code dealing with the menu item labels. This requires more changes
than just modifying GetLabelText() to use some wxStrip_NoCJKMnemonics
flag, but is more compatible as it's not impossible that some existing
code using wxStripMenuCodes() could be broken by this change too, while
now the existing behaviour is preserved.

Also improve wxStrip_XXX documentation.

Closes https://github.com/wxWidgets/wxWidgets/pull/1439

See #16736.

Closes #18452.

interface/wx/utils.h

@@ -682,14 +682,54 @@ void wxSetDisplayName(const wxString& displayName);
 */
 enum
 {
-    // strip '&' characters
+    /**
+        Strip '&' characters.
+
+        This flag removes all the ampersands before another character and
+        replaces double ampersands with a single one.
+     */
     wxStrip_Mnemonics = 1,
 
-    // strip everything after '\t'
+    /**
+        Strip everything after '\t'.
+
+        This flags removes everything following the last TAB character in the
+        string, if any.
+     */
     wxStrip_Accel = 2,
 
-    // strip everything (this is the default)
-    wxStrip_All = wxStrip_Mnemonics | wxStrip_Accel
+    /**
+        Strip everything looking like CJK mnemonic.
+
+        CJK (Chinese, Japanese, Korean) translations sometimes preserve the
+        original English accelerator or mnemonic in the translated string by
+        putting it after the translated string in parentheses, e.g. the string
+        "&File" could be translated as "<translation-of-word-file> (&F)".
+
+        This flag strips trailing "(&X)" from the string.
+
+        @since 3.1.3
+     */
+    wxStrip_CJKMnemonics = 4,
+
+    /**
+        Strip both mnemonics and accelerators.
+
+        This is the value used by wxStripMenuCodes() by default.
+
+        Note that, despite the name, this flag does @e not strip all, as it
+        doesn't include wxStrip_CJKMnemonics for compatibility.
+     */
+    wxStrip_All = wxStrip_Mnemonics | wxStrip_Accel,
+
+    /**
+        Strip everything from menu item labels.
+
+        This flag is used by wxWidgets internally and removes CJK mnemonics
+        from the labels too, in addition to the usual mnemonics and
+        accelerators. It is only suitable for use with the menu items.
+     */
+    wxStrip_Menu = wxStrip_All | wxStrip_CJKMnemonics
 };
 
 /**