Make Feathers only add `InputDispatchPlugin` when it's not already added

[janhohenheim]

What problem does this solve or what need does it fill?

InputDispatchPlugin is not a default plugin, but plenty of plugins need it. So naturally, FeathersPlugins adds it, as it doesn't work without it. But the popular and soon-to-be upstreamed bevy_ui_text_input also adds it!. Combining both thus means you get a runtime panic because of duplicate plugins.
Fortunately, FeathersPlugins is a plugin group, so we can work around this:

FeathersPlugins
    .build()
    // InputDispatchPlugin is also added by TextInputPlugin
    .disable::<InputDispatchPlugin>()
    // Breaks input focus for some reason?
    .disable::<TabNavigationPlugin>(),

but that's not very user friendly

What solution would you like?

Use plugin dependencies at home: check if the plugin already exists before adding it

What alternative(s) have you considered?

• Leave it
• Document this better maybe?
• Wait for real plugin deps
• Tell bevy_ui_text_input to do this instead

Additional context

As requested by Alice ;)

[alice-i-cecile]

#69 sure would be nice here.

[alice-i-cecile]

Ugh, that solution will break the PluginGroup here. Let me poke at a better solution.