Replace wxLocaleIdent ctor from language with FromTag()

This static function parses a subset of the language tags described in
BCP 47 (see https://www.rfc-editor.org/rfc/bcp/bcp47.txt).

Use the tag, as specified by this function, rather than the locale
identifiers components under MSW, where this should allow us to use even
locales that can't be described using just language-script-region.

interface/wx/uilocale.h

@@ -195,25 +195,66 @@ public:
 wxString wxGetUIDateFormat();
 
 /**
-    Allows to construct the full locale identifier in a portable way.
+    Represents a locale in a portable way.
 
-    Parts of the locale not supported by the current platform (e.g. modifier under non-Unix platforms) are ignored.
-    The remaining parts are used to construct a string uniquely identifying the locale in a platform-specific name.
+    There are two possible ways to construct wxLocaleIdent:
 
-    Usage example:
+        - You can either use fromTag() to create it from a string in the form
+          @code language ["-" script] ["-" region] @endcode, corresponding to
+          the subset of BCP 47 (https://www.rfc-editor.org/rfc/bcp/bcp47.txt)
+          syntax.
+        - Or you can create it from the different parts of this string by using
+          the default constructor and then chaining calls to Language(),
+          Region(), Script() and other methods.
+
+    The first method is useful for interoperating with the other software using
+    BCP 47 language tags, while the second one may may result in more readable
+    code and allows to specify Unix-specific locale description parts such as
+    charset and modifier that are not part of the BCP 47 strings.
+
+    Example of using wxLocaleIdent in the second way:
     @code
-      auto loc = wxLocaleIdent("fr").Region("BE").Modifier("euro");
+      auto loc = wxLocaleIdent().Language("fr").Region("BE").Modifier("euro");
     #if defined(__WINDOWS__) || defined(__WXOSX__)
       wxASSERT( loc.GetName() == "fr_BE" );
     #elif defined(__UNIX__)
       wxASSERT( loc.GetName() == "fr_BE@euro" );
     #endif
     @endcode
+
+    For the first way, it is enough to just write
+    @code
+      auto loc = wxLocaleIdent::FromTag("fr-BE"); // Dash, not underscore!
+    @endcode
+
     @since 3.1.6
 */
 class wxLocaleIdent
 {
 public:
+    /**
+        Return the locale identifier corresponding to the given BCP 47-like tag.
+
+        The string must contain at least the language part (2 or 3 ASCII
+        letters) and may contain script and region separated by dashes, i.e.
+        all of the following are valid:
+
+            - "mn"
+            - "mn-MN"
+            - "mn-Cyrl-MN"
+
+        Note that while BCP 47 extlangs, variants, extensions, private use and
+        grandfathered tags are currently not directly supported, they may still
+        work for creating wxUILocale on platforms with native support for BCP
+        47 strings.
+
+        If the input argument uses an unrecognized syntax (e.g. is empty), an
+        empty wxLocaleIdent is returned. Of course, even if this function
+        returns a non-empty object, the resulting locale may still be invalid
+        or unsupported, use wxUILocale::IsSupported() to check for this.
+     */
+    static wxLocaleIdent FromTag(const wxString& tag);
+
     /**
         Default constructor creates an empty and invalid locale identifier.
 
@@ -221,18 +262,6 @@ public:
     */
     wxLocaleIdent();
 
-    /**
-        Constructor with language.
-
-        Note that this constructor is non-explicit, allowing to pass just a
-        simple string, such as "en", to functions taking wxLocaleIdent.
-
-        @param language
-            ISO 639 language code.
-            See Language() for more detailed info.
-    */
-    wxLocaleIdent(const char* language);
-
     /**
         Set language.