Tracker: unchecked return value, CID 602465

Check that refs is not NULL and that refs->FindData() succeeds.

Also, explicit NULL check
Tracker: Fix 3 copies, CID 609040-CID 609042

strcpy() => strlcpy()
Tracker: Fix dereference after NULL, CID 601541

_TrackerLaunchDocuments() dereferences refs to copy it, so, we need
check that it isn't NULL first.

Also a few style fixes.
* Name params consistent with TTracker class methods
  ref, message => appRef, refs
* Use NULL instead of 0 for NULL pointer.
Tracker: check if widget == NULL, CID 600509

Add a few helpful comments too.
Tracker: fix memory leak, CID 600996

We now only create the BString if we're going to put it in the list.
Tracker: fix NULL dereference
Tracker: fix unchecked return value, CID 602466
Tracker: fix use after free, CID 610577

This seems to happen when BList got converted to a BObjectList.
The latter frees items when removing them by default while the former
does not.
Tracker: fix uninitialized pointer, CID 991718

fTrashContextMenu was never initialized in the constructor
(looks like it just got overlooked since all other pointers were.)
Tracker: Check if Submenu() is NULL. CID 601585
Tracker: tiny style fix
Tracker: fix unchecked return value. CID 602487
Tracker: tiny style fix, 80 char limit
Tracker: fix unchecked return value. CID 602488
Tracker: fix uninitialized members, CID 991721
Tracker: fix uninitialized members, CID 610132
Tracker: fix uninitialized members, CID 610133
Tracker: Initialize pointers, CID 610135-610136
Tracker: style fixes, whitespace only
Tracker: Set default values in case of error

...for BColumn. Fixes CID 602478 and CID 602479