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
```
The README.md contains the warning that used to be in lib.rs.
Add README.md files to all internal crates
... pointing to the official public crate to use instead.
Rename internal crates
fixup: README files
fixup rename
We want to be able to put existing functionality behind a feature flag while keeping
the semver compatibility.
This is only possible if that new feature flag is enabled by default, but this is not
working if the users have done `default-features = false` in their Cargo.toml.
So we add new `compat-x-y-z` feature that is mandatory to have and which is
enforced with a `compile_error!`
Now, users that whishes to not have the default features must enable it explicitly.
Say we want only x11 but not qt and wayland, the user will do
```toml
sixtyfps = { version = "0.2", default-features = false, features = ["x11", "compat-0-2-0"] }
```
Now, imagine that in the version 0.2.3, we put the SVG support behind a feature flag.
we will do this in out Cargo.toml:
```toml
[features]
default = ["compat-0-2-0", "x11", "wayland"]
compat-0-2-0 = ["compat-0-2-3", "svg"]
compat-0-2-3 = []
svg = [...]
...
```
That way, the svg feature will be enabled by default for all the users who used previous version
of SixtyFPS, and people that want to disable "svg" can just change from compat-0-2-0 to
compat-0-2-3 in their Cargo.toml
Move "internal" crates into the `internal` directory. This first batch
includes most of sixtyfps_runtime but leaves the rendering backends
alone for now.
pre-commit applied some cleanups to the moved files:
- Consistent newline at end of file policy
- trimming trailing whitespace
- Formatting Cargo.toml files.
The ffi function wrapper need to be changed because in the 2021 edition, when
the capture is moved into the closure, it moves the individual field instead
of the whole wrapper. But we need to move the whole wrapper because the Drop
of the wrapper will delete the C++ closure, and we don't want to call the
closure after it is deleted.
