Fix sorting by translation on Windows

Add ConvertToSortKey() overload that accepts rvalue references, so that
temporary wxString objects, such as GetTranslation() return value, have
their string data copied into the ICU buffer.

src/cat_sorting.h

@@ -106,6 +106,11 @@ class CatalogItemsComparator
         }
     }
 
+    static str::UCharBuffer ConvertToSortKey(const wxString&& a)
+	{
+		return std::move(ConvertToSortKey(a).ensure_owned());
+	}
+
 private:
     const Catalog& m_catalog;
     SortOrder m_order;

src/str_helpers.h

@@ -203,16 +203,17 @@ class UCharBuffer
     int32_t capacity() { return m_capacity; }
 
     /// Ensure the buffer has a deep copy of the data, if it's not already owned
-    void ensure_owned()
+    UCharBuffer& ensure_owned()
     {
         if (m_owned)
-			return;
+			return *this;
         if (m_capacity == -1)
             m_capacity = u_strlen(m_data) + 1;
         auto copy = new UChar[m_capacity];
         memcpy(copy, m_data, m_capacity * sizeof(UChar));
         m_data = copy;
         m_owned = true;
+		return *this;
 	}
 
 private:
@@ -280,11 +281,21 @@ inline UCharBuffer to_icu(const wxString& str)
     return to_icu(str.wx_str());
 }
 
+inline UCharBuffer to_icu(const wxString&& str)
+{
+    return std::move(to_icu(str.wx_str()).ensure_owned());
+}
+
 inline UCharBuffer to_icu(const std::wstring& str)
 {
     return to_icu(str.c_str());
 }
 
+inline UCharBuffer to_icu(const std::wstring&& str)
+{
+    return std::move(to_icu(str.c_str()).ensure_owned());
+}
+
 inline UCharBuffer to_icu(const std::string& str)
 {
     return to_icu(str.c_str());