Multiple-select should use ctrl

[Daniel-Beardsmore]

Selecting multiple keys requires shift. Multiple-select should be ctrl or cmd; shift typically indicates range selection, and ctrl/cmd/adjust indicates discrete selection. Shift-click would for example be used to select a whole row of keys, although in kbedit it's going to be tricky to define a 100% predictable behaviour for range selection.

Also, ctrl+click should toggle selection (currently shift-click can only select, not deselect)