Debugger symbols: Add new symbol type: Notes.. Notes are for naming single instructions, or small groups of instructions.

Notes are separate from function symbols, and can be searched separately.
Unlike functions, notes of different length can overlap each other.
In the instruction window, a note will always display over the function symbol.

Source/Core/Common/SymbolDB.cpp

@@ -51,6 +51,7 @@ void SymbolDB::Clear(const char* prefix)
 {
   // TODO: honor prefix
   m_functions.clear();
+  m_notes.clear();
   m_checksum_to_function.clear();
 }