- 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.

fixes #435

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'

Suggested-by:

Clang-Tidy: Use auto when initializing with new to avoid duplicating the type name

This could actually change the behaviour here (could, because I don't
know if we can hit this special case) but this should hopefully the
correct way of doing this.

There's some detailed explanation of the difference here:
https://doc.qt.io/qt-5/qstring.html#arg-14

> This is the same as str.arg(a1).arg(a2), except that the strings
> a1 and a2 are replaced in one pass. This can make a difference if
> a1 contains e.g. %1:

```
QString str;
str = "%1 %2";

str.arg("%1f", "Hello");        // returns "%1f Hello"
str.arg("%1f").arg("Hello");    // returns "Hellof %2"
```

Suggested-by:

Clazy: Use multi-arg instead

Suggested-by:

Clang-Tidy: 'find' called with a string literal consisting of a
single character; consider using the more effective overload
accepting a character

All suggested by:

Clang-Tidy: The parameter 'X' is copied for each invocation
but only used as a const reference; consider making it a const reference

Suggested by:

Clazy: Use the static QFileInfo::exists() instead. It's documented to be faster.