Markers: add delete confirmation property

Defaults so that only X (backspace for industry
compatible keymap) prompts for deletion,
whereas DEL does not show the confirmation popup.

Previously, markers always showed a confirmation. Now add a property to
allow disabling this behavior. A similar change (for deleting keyframes)
was previously done in D9651.

This also makes the default keymap for marker deletion consistent with
the common delete operators (such as object mode).