Support Gnome 51

Mutter 51 has removed the `experimental-features` key in the `org.gnome.mutter` schema (and introduced a new `org.gnome.mutter.experimental` schema, but that doesn't matter here).
Additionally, some settings, such as fractional scaling, are on by default and no longer experimental.
We should handle these gracefully so tilingshell works across multiple versions

Start by locating the GSettings handling for the org.gnome.mutter schema and the code that reads experimental-features or fractional-scaling settings. Compare behavior with GNOME 51, where the key is absent and some settings are enabled by default. Done means tilingShell handles GNOME 51 and earlier versions without errors or incorrect assumptions about experimental settings.

Written by the indexing model from the issue text.