Follow-up to #30565
This PR fixes the default settings values for the `DiagnosticsSettings`.
The issue here was that due to the `#[derive(Default)]`, `button` would
be false by default, which unintentionally hid the diagnostics button by
default. The `#[serde(default = `default_true`)]` would only apply iff
the diagnostics key was already present in the user's settings. Thus, if
you have
```json
{
"diagnostics": {...}
}
```
in your settings, the button would show (given it was not disabled).
However, if the key was not present, the button was not shown: Due to
the derived default for the entire struct, the value would be false.
This PR fixes this by implementing the default instead and moving the
`#[serde(default)]` up to the level of the struct.
I also did the same for the inline diagnostics settings, which already
had a default impl and thus only needed the serde default on the struct
instead of on all the struct fields.
Lastly, I simplified the title bar settings, since the serde attributes
previously had no effect anyway (deserialization happened in the
`TitlebarSettingsContent`, so these attributes had no effect) and we can
remove the `TitlebarSettingsContent` as well as the attributes if we
implement a proper default implementation instead.
Release Notes:
- Fixed the diagnostics status bar button being hidden by default.
09503333af