Improve readability of UiItem property names

[magnesj]

Convert

std::optional<bool> m_isHidden;
std::optional<bool> m_isTreeHidden;
std::optional<bool> m_isTreeChildrenHidden;

to

std::optional<bool> m_isVisible;
std::optional<bool> m_isTreeVisible;
std::optional<bool> m_isTreeChildrenVisible;