Add a 'Delete Confirmation' operator property to the 'Delete Marker'
operator. This determines whether the user is asked to confirm the
deletion or not.
Defaults so that only {key X} ({key Backspace} for industry compatible
keymap) prompts for deletion, whereas {key Del} does not show the
confirmation popup.
This also makes the default keymap for marker deletion consistent with
the common delete operators (such as objects and keyframes).
Reviewed By: sybren
Differential Revision: https://developer.blender.org/D13818
4344b2bf19