Implement smarter detection of windowing environments.

This patch centralises the loading of windowing environments, in order
to ensure that by default only a single module is loaded, rather than
the current code which can load multiple potentially incompatible
modules; and in order to provide the user with more control over the
loading of windowing modules.

The patch introduces a new str-list option `windowing_modules` which
defines an ordered list of windowing modules to attempt to load. Modules
are loaded in the order specified in the list until a module loads
without error, at which point the process finishes.

When loaded each windowing module tests the environment to determine
whether it should load (e.g. the tmux module tests to see if it's being
run within a tmux session), and if it determines that it should then it
completes its loading without error. If it doesn't detect an appropriate
environment then it returns an error, and the module loading logic tries
the next module.

The user can override the default `windowing_modules` list to specify
their preferred modules (i.e. they can put kitty ahead of tmux if that's
their preference, or they can leave out the x11 modules alltogether). In
addition, if the `windowing_modules` option is an empty list this
bypasses the environment detection logic completely, and allows the
modules to be loaded manually - this allows a user to replace the
windowing module loading logic with their own manual set up.

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