Add vertical text alignment to Label controls

Add a read/write Label.VerticalTextAlignment property so desktop labels can align their text to the top, middle, or bottom of the available label area. Use the existing TextAlignment.Top, TextAlignment.Middle, and TextAlignment.Bottom values, and expose the setting in Studio's Inspector as well as in code.

This is useful for explanatory text beneath a heading: developers can reserve a taller label for longer translations while keeping shorter text close to the heading. Vertical alignment should position the entire text block, including wrapped multiline text, within the label's existing bounds.

The default is TextAlignment.Middle, preserving the appearance of existing labels and saved projects.