Remove "const" from a bunch of Grob methods
Remove "const" from the fundamental property access methods like internal_get_property, and walk through the consequences. These methods can trigger callbacks, which can perform all sorts of side effects (like suicide), which is not const even in the logical (vs. physical) sense. Use const_cast in cases where const makes sense, such as accessing the fundamental properties like meta for which callbacks are not expected.
Fixes #5349 (closed)