Check errors with derive macro attributes

[piegamesde]

Bug description

The CompositeTemplate macro (and probably others too) currently do not sufficiently check the validity of present attributes. So instead of getting a compile error, things will silently fail.

#[derive(CompositeTemplate, Default)]
#[template(file = "../../res/window.ui")]
pub struct CustomWidget {
	#[template]
    header: adw::HeaderBar,
}

[sdroege]

CC @jf2048

[jf2048]

I am currently working on another library for parsing these attributes that does a lot more validations, would like to switch everything over to that eventually so we can get rid of attribute_parser