applets/notifications: Embellish for critical notifications

This is a perhaps less hideous but also less visible alternative to !1098 (closed). As a bonus, it does not require any API changes in frameworks, so frameworks/plasma-framework!348 (closed) could be abandoned.

We get intermittent user complaints and bug reports about notifications not being visible enough, especially on large or cluttered screens.

This commit attempts to remedy the situation by tinting the header for on the left side for critical notifications using the color scheme's "warning" color. Only critical notifications receive this treatment since they're the only ones that the user really does need to see.

BUG: 420541 FIXED-IN: 5.24

Showing long text and short text:

cc @teams/vdg @teams/usability