Fixes #2824. Add SearchText property to ComboBox

[MaciekWin3]

Fixes #2824 - Added SearchText property to ComboBox

Also, the issue of resetting the text in the search field was resolved. For instance, when utilizing a constructor to provide text that I presumed should serve as the default content in the search field, the text was getting reset upon adding the ComboBox to the Window. Moreover, this text was also reset when a button was pressed and no item was selected from the ComboBox. This behavior was quite annoying.

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[tig]

Please create another PR that applies this fix to v2 as well. Thanks.