crates.io won't let us upload a feature with dots in it:
```
Uploading slint-interpreter v0.3.0 (/home/olivier/slint/internal/interpreter)
error: failed to publish to registry at https://crates.io
Caused by:
the remote server responded with an error: invalid upload request: invalid value: string "compat-0.3.0", expected a valid feature name at line 1 column 2254
```
There's a memory leak that causes the created canvas elements not be
deleted. I've tried a few thing such as explicitly hiding the window
(destroying the canvas element instance refs and gl resources we keep in
Rust), but it's not enough - somewhere there remains a circular
reference. Possibly between some of the closures installed in Rust and
DOM elements they are installed on as event handlers.
I also tried using the wasm-bindgen support for weak refs, but no luck.
So instead, to plug the leak, this patch introduces the re-use of the
HTML canvas element in a way that is similar to how the preview works in
vs code's live preview.
I verified that no GL resources or new canvas elements are leaked in
Chrome's heap profiler via snapshot comparison and filtering for the
corresponding DOM element types.
We don't need to compile cpp, qttypes, and all such dependencies to only
have the qt backend's buildscript ignore the qt compilation error and
fallback to the dummy no_qt backend.
Only the last shown preview was interactive. This was because we started the winit event loop for each preview,
which kind of invalidates the previous instance in winit.
Instead, create all components, call show() on each instance to register the window with winit
and spin the event loop with its exception magic only once.
