Tracker: obligatory round of style fixes

* Explicit NULL checks
* whitespace
* single line if gets no {}'s
*
Tracker: Make sure count > 0

... in a more stylish way, no negative counts allowed.
Tracker: Don't crash clicking Open with... menu item

This bug was introduced in hrev47498

The reason Tracker crashed was because OpenWithContainerWindow
had a NULL TargetModel() which we were trying to dereference.

Fix this by creating an empty model in OpenWithContainerWindow. Add an
ASSERT to check that the TargetModel() is not NULL in the future.

Another way to fix this bug would have been to check that TargetModel() wasn't
NULL each time before we use it. I didn't go with that solution because we
assume TargetModel() is not NULL in a lot of places so it would be a lot of work
to check them all. I think it's better to give OpenWithContainerWindow a dummy
model even though it doesn't make sense for it to have one just so that we don't
crash when we try to dereference the pointer.