Add font-size and font-weight properties to CheckBox

[dfaure-kdab]

Expose font-size and font-weight as input properties on CheckBox across
all non-native styles (fluent, material, cupertino, cosmic), defaulting
to each style's existing font settings. This matches the pattern already
used by TextEdit, TextInput, and LineEdit.

These properties have no effect when using the Qt style, where the
entire widget (including text rendering) is delegated to NativeCheckBox
which paints via QStyle::drawControl.

[ogoffart]

We need to discuss if we really want this. We probably do, but then we should have that consistently to all widgets.

I think @tronical have been wanting to have a more general property<Font> font: { size: ..., weight: ... }; that would reduce the need to add way too many property.

Relates to #4372

[dfaure-kdab]

Yeah. This solution is consistent with TextEdit, TextInput, and LineEdit, but means it's still missing from ComboBox etc.

A font property would make sense but only if one can specify the size without having to specify everything (especially hardcoding a font family name).

[tronical]

Yeah. This solution is consistent with TextEdit, TextInput, and LineEdit, but means it's still missing from ComboBox etc.

Agreed.

A font property would make sense but only if one can specify the size without having to specify everything (especially hardcoding a font family name).

Yes. The idea is to carry a Font struct in the implementation and additionally offer font-* properties for convenience (the example you described).

IMO this PR adds the API we want, and I want to make the implementation more lightweight.