Various deprecation warning fixes

See the commit messages.

This does various cleanups on the code.

• Fixes a few deprecation warnings from recent versions of GLib (but not exhaustively).
• Improves the GObject property handling in a few classes to reduce string copies and unnecessary GObject::notify emissions (but not exhaustively).
• Removes some GObject boilerplate.
• Fixes some small leaks noticed while valgrinding basic startup of gnome-session.

There’s no particular logic behind which parts of the code I’ve fixed up and which parts I haven’t touched. I was just fixing these bits up while working on something related (which I’ll post in a separate MR).

There should be no need to exhaustively review the “Modernise GObject private data handling” changes. They were done as a find/replace of (e.g.) s/client->priv/priv/ with some manual fixups afterwards to fix remaining compiler warnings.

These commits should introduce no functional changes to the code.

Signed-off-by: Philip Withnall pwithnall@endlessos.org