- Set PNG compression to maximum
- Don't upload thumbnail if it is not at least 10% smaller than  the
  original

fixes #435

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.

Either by accepting an invite or manually using /converttodm and revert
with /converttoroom.

fixes #751

This ports the leave room dialog to QML, everywhere. There are now no differences between the various leave dialogs.