v0.15.0

Changelog

• 6dec878 Updated CHANGELOG
• d02f9d7 Fix AlertDialog broken content when testing in Flet app (#2192)
• dfe9b4e Bump Flet version to 0.15.0
• 38112d6 add Option.visible (#2190)
• 6dd23da Additional props (#2182)
• f921a6d ExpansionPanel and ExpansionPanelList controls (#2169)
• 62f4327 CupertinoCheckBox control, adaptive CheckBox (#2173)