Combo box multi column support

[akleshchev]

A 3p contribution to enchance/fix combo box functionality when it contains multiple columns

Test Plan

Go to XUI Preview and open floater_test_combobox, last combo box should show all strings fully.
Check other combo boxes in the same floater and compare them to release.

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[Dan-Linden]

Passed QA.
Verified on Second Life Release 26.2.0.21655710006 on Win11/OSX in the scope of https://github.com/secondlife/iqa/issues/606

All other combo boxes on this floater work the same as Release 7.2.3.19375695301.