- 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.

This change adds the ability to run Nheko without a secrets storage
service[1]. The secrets are stored in plaintext under the [secrets]
section of nheko.conf. Since this is a "I know what I'm doing" type
of feature, it's disabled by default and can only be enabled with an
hidden setting `run_without_secure_secrets_service=true`.

Fixes issue #705.

[1]: https://freedesktop.org/wiki/Specifications/secret-storage-spec

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