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/DXF/DXFLoader.cpp

@@ -125,18 +125,9 @@ DXFImporter::~DXFImporter() {
 
 // ------------------------------------------------------------------------------------------------
 // Returns whether the class can handle the format of the given file.
-bool DXFImporter::CanRead( const std::string& filename, IOSystem* pIOHandler, bool checkSig ) const {
-    const std::string& extension = GetExtension( filename );
-    if ( extension == desc.mFileExtensions ) {
-        return true;
-    }
-
-    if ( extension.empty() || checkSig ) {
-        const char *pTokens[] = { "SECTION", "HEADER", "ENDSEC", "BLOCKS" };
-        return BaseImporter::SearchFileHeaderForToken(pIOHandler, filename, pTokens, 4, 32 );
-    }
-
-    return false;
+bool DXFImporter::CanRead( const std::string& filename, IOSystem* pIOHandler, bool /*checkSig*/ ) const {
+    static const char *tokens[] = { "SECTION", "HEADER", "ENDSEC", "BLOCKS" };
+    return SearchFileHeaderForToken(pIOHandler, filename, tokens, std::size(tokens), 32);
 }
 
 // ------------------------------------------------------------------------------------------------