Make the modifiers simple booleans in the key event

This will give a nicer API to expose to .60. If the struct weren't
repr(C) then the booleans would be nicely packed, but alas that's not
happening. On the other hand we're not keeping many instances of them
around.

api/sixtyfps-rs/lib.rs

@@ -199,8 +199,6 @@ pub mod re_exports {
     };
     pub use sixtyfps_corelib::input::{
         FocusEvent, InputEventResult, KeyEvent, KeyEventResult, KeyboardModifiers, MouseEvent,
-        ALT_MODIFIER, CONTROL_MODIFIER, COPY_PASTE_MODIFIER, LOGO_MODIFIER, NO_MODIFIER,
-        SHIFT_MODIFIER,
     };
     pub use sixtyfps_corelib::item_tree::{
         item_offset, visit_item_tree, ItemTreeNode, ItemVisitorRefMut, ItemVisitorVTable,