decor: ellipsize long window titles

[sanyarnd]

Long window titles can currently wrap or run into the titlebar buttons because the Pango layout has a width but no single-line constraint.

This keeps the existing horizontal and vertical alignment settings, while limiting the title to one line and ellipsizing it at the end when it does not fit. Short titles render as before.

Testing:

• The changed translation unit passes a C++ syntax check.
• Full local configuration is currently blocked because the host provides Wayland 1.22 while the bundled wlroots 0.20 requires Wayland 1.24.
• Manual checks are still needed for left-, center- and right-aligned titles.