ttk_menu and cursor icon

[benini]

It would be nice to change the mouse cursor to a hand icon when left-clicking shows a menu.
For example when the mouse is over the evaluation bar, or in the database switcher when hovering a database block.
Maybe even in the PGN window to indicate that clicking on the header toggle the view between all/short.

• ttk_menu - macos: menu's color on macos cannot be changed. It works well because it use the system defaults. Some images don't work well with the dark theme, i.e the main board menu's ones -> remove the images
• ttk_menu - linux: use a fixed image for cascade submenu. It will never use a system default like windows or macos, maybe it is accepteable? We can change the top menu colors according to the theme
• ttk_menu - win: cascade problem with a fixed black image plus an ugly "border" because only the background of the labels is changed. Maybe they will fix it in future releases of Tk.