Restore Alt+drag rectangular selection in the editor. #9864 #10029 (#10043)

The fix for #9570 (stop Alt+F5 showing a crosshair cursor) changed the
rectangular-selection eventFilter to require Alt+Ctrl and moved the
crosshair cue to Control. That broke the long-standing Alt+drag block
(column) selection and left an inconsistent Ctrl crosshair cue.

Restore the default rectangularSelection() (Alt+drag) and drop
crosshairCursor entirely - the crosshair-on-Alt was exactly the artifact
#9570 wanted gone, and CodeMirror's crosshair cannot be limited to an
active drag. This brings back block selection (#9864, #10029) while
keeping #9570's intent (no crosshair on the Alt+F5 shortcut).