This PR adds the following lint rules to our list:
```
always_put_required_named_parameters_first
avoid_multiple_declarations_per_line
avoid_positional_boolean_parameters
avoid_returning_null_for_void
avoid_returning_this
avoid_unnecessary_containers
enable_null_safety
library_private_types_in_public_api
no_leading_underscores_for_library_prefixes
no_leading_underscores_for_local_identifiers
prefer_null_aware_method_calls
tighten_type_of_initializing_formals
unnecessary_late
use_setters_to_change_properties
```
And these rules were considered, and some changes were made according to
them as a clean-up, but in many places they didn't make sense
(`prefer_asserts_with_message` I would have included, but there were too
many places that needed to be changes):
```
collection_methods_unrelated_type
prefer_asserts_with_message
avoid_renaming_method_parameters
```
This PR is the second in a series of refactors that aim to simplify event handling in Flame. The approach is as follows:
Added the MultiTapDispatcher component, which contains the logic that used to be within the HasTappableComponents mixin. This component is internal; it mounts to a FlameGame directly, and ensures that it is a singleton.
Whenever any TapCallbacks component is added to a game, it automatically adds the MultiTapDispatcher component (unless there is already one), which in turn registers a tap gesture detector with GestureDetectorBuilder and rebuilds the game widget.
The end result is that now in order to make a component tappable you only need to add the TapCallbacks mixin to that component, everything else will be handled by the framework.
Consequently, the HasTappableComponents mixin is now empty and marked as deprecated.
