fix: use text widget instead of custom tab_text

[hojjatabdollahi]

Draft while waiting for pop-os/libcosmic#1140

Ellipsizes the window title in SSD and tab text for stacked windows.

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[git-f0x]

Actually, specifying the font with .font(self.font) might be needed, since it's used in tabs.rs to set the font to semibold. Sorry about that.

[git-f0x]

Seems to match previous behavior (other than having ellipsize), with the text of the active tab becoming semibold and accent-colored, and the close button disappearing for non-active tabs when the tab is narrow enough.
A potential future improvement could be to not show any text (only icon) for non-active tabs if it would end up being only "..." (at really narrow widths, only ".." is visible), using that extra width for the active tab. But that's out of scope of this PR.

[hojjatabdollahi]

superseded by #2180