Draft: Optimize file selection performance by replacing GList with GHashTable (#1371)

Replace the GList-based selected_files container with a GHashTable to improve performance when handling large file selections. This addresses performance bottlenecks described in issue #1371.

Key changes:

• Convert selected_files from GList* to GHashTable* in ThunarStandardViewPrivate
• Use g_hash_table_new_full() for initialization with proper cleanup
• Replace all list iteration with hash table iteration using GHashTableIter
• Optimize drag-and-drop by building file lists directly from hash table
• Update statusbar logic to use O(1) size checks instead of g_list_length()
• Improve selection transfer between views using hash table operations