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/SMD/SMDLoader.cpp

@@ -101,9 +101,9 @@ SMDImporter::~SMDImporter() {
 
 // ------------------------------------------------------------------------------------------------
 // Returns whether the class can handle the format of the given file.
-bool SMDImporter::CanRead( const std::string& pFile, IOSystem* /*pIOHandler*/, bool) const {
+bool SMDImporter::CanRead( const std::string& /*pFile*/, IOSystem* /*pIOHandler*/, bool) const {
     // fixme: auto format detection
-    return SimpleExtensionCheck(pFile,"smd","vta");
+    return false;
 }
 
 // ------------------------------------------------------------------------------------------------