usability: menus should use cursor path detection to avoid opening submenus unintentionally, on platforms where it's possible

Some explanation:

http://www.asktog.com/columns/022DesignedToGiveFitts.html answer to question 6
https://bugzilla.mozilla.org/show_bug.cgi?id=5927 comment 34
http://bjk5.com/post/44698559168/breaking-down-amazons-mega-dropdown

Qt uses native menus where possible, but on Linux, Qt provides the actual menu implementation. So we have the opportunity to make the menu implementation best-in-class on Linux at least, by implementing this trapezoidal zone where you can drag over diagonally to items on a submenu without accidentally triggering other menu items on the way over, and also without adding too much delay for that purpose. Maybe it can be done on some other platform too?