- If an inline image has specified a height, add parameters to the
  image:// URI.
- Add scaled to the parameters, the images would be cropped otherwise.
- Extract the height from image:// URI and use it for requestSize.
- Use scaledToHeight instead of scaled.

Might fix #844

See
https://code.qt.io/cgit/qt/qtdeclarative.git/commit/examples/quick/imageresponseprovider/imageresponseprovider.cpp?h=5.15&id=b1f238568214e6587b829d6695677e55a99b1d40
for context.

relates to #845

It seems like handling the message in a blocking manner is a no-go. I
have no idea how to fix that, so just use a queued connection for now...
(ASAN does not cooperate and just hides the crash D:)

fixes #842

fixes #435

fixes #824

fixes #519

This warns when you the return value of those functions isn't used.

Suggested-by: Clang-Tidy: Function X should be marked [[nodiscard]]

Suggested-By:

Clang-Tidy: Single-argument constructors must be marked explicit to
avoid unintentional implicit conversions

More info: https://clang.llvm.org/extra/clang-tidy/checks/google-explicit-constructor.html

Suggested-by:

Clang-Tidy: The 'empty' method should be used to check for emptiness instead of 'size'