Apply new color_scheme safely
Apply new color_scheme safely

For new created color_schemes, it takes Sublime a tick or two to register
them. We can check for 'resources' using `sublime.load_resource`. Hence,
the strategy is simply to check before actually applying a color_scheme.

- It is important that the first call to `try_apply_theme` is not debounced, in
case we did not create but just update a theme. Otherwise it may shortly
flicker for no reason.

- It is also important that the first *retry* delays with 0, t.i. right as the
next task bc that is usually enough.

After that we properly bounce off after each try.

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