Clarify highlight list item summary (icons, matching all)

Test case

Steps

1. Open overflow menu → Core Settings → Highlights
2. Add a new highlight rule
   • Name set to an empty string
   • Sender to test
   • Channel to #test (or the name of a buffer)
3. Tap Save

Expected

Highlight Rules list indicates in some way that the highlight rule matches all text, e.g. a placeholder in italics. Sender and channel are distinguished.

Example:

Any message content

[person icon] test

#test

This may make it more obvious when hunting down a rogue highlight/highlight ignore rule that matches everything, as well as clarifying between sender and channel matching.

Actual

Highlight Rules list shows a blank subject message.

Example:

test

#test

Additional

This is a subjective change; leaving it blank works for glance-ability, too.

Quassel itself doesn't provide a placeholder message, mostly since I didn't know how to do that in Qt's list view at the time. The tooltip text does mention "leave blank to match any phrase" (though I'm not sure who reads the tooltips).