Use an editable font dropdown for Inspector FontName properties

Several desktop controls expose FontName in the Window Designer Inspector. This is currently edited as a free text field, which makes valid font names hard to discover and easy to mistype.

Change the Inspector editor for FontName to an editable dropdown. The dropdown should list fonts available to the current Studio host, include a default/empty option, and still allow arbitrary typed values. This gives users a better normal workflow while preserving portability for projects that target another platform, another machine, or future bundled project fonts.

Expected user benefit:

• Users can choose installed fonts without knowing exact family names.
• Existing projects with unavailable font names are preserved instead of silently changed or rejected.
• FontName remains compatible with deployment targets whose fonts differ from the IDE host.