storing selected paths should consider time that paths were selected

[ad510]

See the unitsel branch for early work on this. A side effect of this is that making new paths will select the new paths as well, because the program will no longer distinguish which is the "original" and which is the "new" path. Consider using the KeyValuePair struct (probably a bad idea but throwing it out there) if we don't want to make a new class or struct. Decide whether to store commanded units this way in UnitCmdEvt as well.

[ad510]

Fixing this also fixes a bug that occurs if you select 2 buildings, the first one is destroyed, then you try to make a worker or marine.