App: Fix output string to XML

Not all unicode characters are allowed as XML output. When writing disallowed characters the SAX parser throws an exception
when loading a project file that results into a broken document and thus to a possible loss of data.

This PR replaces all disallowed characters with an underscore and prints a warning.

This fixes https://github.com/FreeCAD/FreeCAD/issues/22123
Note: It does not fix an already corrupted project file.

src/Base/Persistence.cpp

@@ -24,7 +24,11 @@
 #include "PreCompiled.h"
 
 #ifndef _PreComp_
+#include <algorithm>
+#include <array>
 #include <cassert>
+#include <codecvt>
+#include <locale>
 #endif
 
 #include <zipios++/zipinputstream.h>
@@ -112,6 +116,41 @@ std::string Persistence::encodeAttribute(const std::string& str)
     return tmp;
 }
 
+// clang-format off
+// https://www.w3.org/TR/xml/#charsets
+static constexpr std::array<std::pair<char32_t, char32_t>, 6> validRanges {{
+    {0x9, 0x9},
+    {0xA, 0xA},
+    {0xD, 0xD},
+    {0x20, 0xD7FF},
+    {0xE000, 0xFFFD},
+    {0x10000, 0x10FFFF},
+}};
+// clang-format on
+
+/*!
+ * In XML not all valid Unicode characters are allowed. Replace all
+ * disallowed characters with '_'
+ */
+std::string Persistence::validateXMLString(const std::string& str)
+{
+    std::wstring_convert<std::codecvt_utf8<char32_t>, char32_t> cvt;
+    std::u32string cp_in = cvt.from_bytes(str);
+    std::u32string cp_out;
+    cp_out.reserve(cp_in.size());
+    for (auto cp : cp_in) {
+        if (std::any_of(validRanges.begin(), validRanges.end(), [cp](const auto& range){
+                return cp >= range.first && cp <= range.second;
+            })) {
+            cp_out += cp;
+        }
+        else {
+            cp_out += '_';
+        }
+    }
+    return cvt.to_bytes(cp_out);
+}
+
 void Persistence::dumpToStream(std::ostream& stream, int compression)
 {
     // we need to close the zipstream to get a good result, the only way to do this is to delete the