Use bitfield instead of bools in Response and Sense (#5556)

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to test and add commits to your PR.
* Remember to run `cargo fmt` and `cargo clippy`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->

Closes <https://github.com/emilk/egui/issues/3862>.

Factoring the `bool` members of `Response` into a bitfield, the size of
`Response` is now 96 bytes (down from 104).

I gave `Sense` the same treatment, however this has no effects on
`Response` due to padding. I've decided not to pursue `PointerState`, as
it is quite large (_many_ members that are sized and aligned to
multiples of 8 bytes), so I don't expect any noticeable benefit from
making handful of `bool`s slightly leaner.

In any case, the changes to `Sense` are already quite a bit more
intrusive than those to `Response`.
The previous implementation overloaded the names of the attributes
`click` and `drag` with similarly named methods that _construct_ `Sense`
with the corresponding flag set. Now, that the attributes can no longer
be accessed directly, I had to introduce methods with new names
(`senses_click()`, `senses_drag()` and `is_focusable()`). I don't think
this is the cleanest solution: the old methods are essentially redundant
now that the named constants like `Sense::CLICK` exist. I did however
not want to needlessly break backwards compatibility.
I am happy to revert it (or go further 🙂) if there are concerns.

Cargo.toml

@@ -73,6 +73,7 @@ ahash = { version = "0.8.11", default-features = false, features = [
     "std",
 ] }
 backtrace = "0.3"
+bitflags = "2.6"
 bytemuck = "1.7.2"
 criterion = { version = "0.5.1", default-features = false }
 dify = { version = "0.7", default-features = false }