This commit wires up the as-yet-unused "remove alternate" tool action
and uses it to select copper connections (normally 'U') before deleting
segments.
THis also reverses the sense of Delete and Backspace (Delete used to be
'remove' and Backpace was 'remove alt', now it is reversed). This means
that backspace is the key that removes a segment and Delete removes the
track. This is the same as legacy behaviour. Other delete actions are,
for now, the same between Delete and Backspace.
Allow selectConnection/Copper/Net on multiple items
The previous behaviour was to act on only the first item in the
selection. The new behaviour is to act on every eligible item (in this
case, tracks and vias).
