Simplified importer search and fixed a few bugs

The search for a matching importer had a few issues, see #3791. There were two different mechanisms to determine whether an importer accepts a specific file extension:
1. `aiImporterDesc::mFileExtensions`, which was forwarded to the UI via `BaseImporter::GetExtensionList()`.
2. `BaseImporter::CanRead()` when called with `checkSig == false`, which determines whether to actually use that importer.
Both were redundant and got out of sync repeatedly. I removed 2. completely and replaced it with 1., thereby syncing UI/import and shortening all `BaseImporter::CanRead()` implementations.

Further bugfixes:
- fixed glTF2 importer throwing exceptions when checking whether it can load a file
- removed `BaseImporter::SimpleExtensionCheck()` because it is no longer used and had a bug with case sensitivity

Since the `checkSig` parameter in `BaseImporter::CanRead()` is now useless, it can be removed completely. I’m not sure if this would break ABI compatiblity, so I’ll submit it with a later pull request.

code/AssetLib/X/XFileImporter.cpp

@@ -89,17 +89,9 @@ XFileImporter::~XFileImporter() {
 
 // ------------------------------------------------------------------------------------------------
 // Returns whether the class can handle the format of the given file.
-bool XFileImporter::CanRead( const std::string& pFile, IOSystem* pIOHandler, bool checkSig) const {
-    std::string extension = GetExtension(pFile);
-    if(extension == "x") {
-        return true;
-    }
-    if (!extension.length() || checkSig) {
-        uint32_t token[1];
-        token[0] = AI_MAKE_MAGIC("xof ");
-        return CheckMagicToken(pIOHandler,pFile,token,1,0);
-    }
-    return false;
+bool XFileImporter::CanRead( const std::string& pFile, IOSystem* pIOHandler, bool /*checkSig*/) const {
+    static const uint32_t token[] = { AI_MAKE_MAGIC("xof ") };
+    return CheckMagicToken(pIOHandler,pFile,token,std::size(token));
 }
 
 // ------------------------------------------------------------------------------------------------