fix: fix changelog, warnings and lints

[w-lfchen]

Description

While working on #1144 I noticed some new warnings when compiling. I'm assuming some warnings were stabilized or added.
This pr fixes these warnings, yielding a slight performance increase, for more details see below.
These are some breaking internal changes since APIs are modified and some call sites might now require generic annotations.

Replacing the field display_backend in the App struct with PhantomData

This field is never read, producing a warning when compiling with 1.80.0.
It seems to only exist for the type parameter B, which has not been restricted to DisplayBackend even though it probably should be.
By replacing it with a PhantomData, there is no need to pass the DisplayBackend around at all times since all the information this argument provides is already encoded in the generic type parameters.

Removal of field instance_id in the EwwWindow struct

This field is never read, producing a warning when compiling with 1.80.0.
All information it holds is already in the key of the open_windows map in the App struct. Removing it also makes the WindowInitiator's id field obsolete.

Checklist

• I added my changes to CHANGELOG.md, if appropriate.
  I don't think this is necessary here
• I used cargo fmt to automatically format all code before committing

[elkowar]

damn, the way github displays this commit msg confused me for a good second, lmao

Thanks for your cleanup work!