fix: ellipsize headerbar title instead of wrapping

[hojjatabdollahi]

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[git-f0x]

Just a note that on e.g. KDE, the titles are ellipsized in the middle, so maybe it makes sense to do the same in libcosmic.

[hojjatabdollahi]

Interesting. I assume I should do the same for tab_text too. Right?

[git-f0x]

Tab text might be different (at least for comp), since it often just ends with the name of the app, which isn't too useful.
I added ellipsizing at the end for the App Tray, since the info at the start is more important than seeing e.g. xyz...(Fi)les.

[hojjatabdollahi]

Maybe the ui/ux team can chime in.

I don't think ellipsize::middle makes a lot of sense, it's not that readable.

[hojjatabdollahi]

We can probably move forward with this PR, now that Maria has Okayed it.