Allow deletion on individual leaks in DisplayActivity

[jrodbx]

Thanks to #1081, I get a bunch of false alarms which I ignore.

Occasionally, I get a leak I want to dig into more, but it's surrounded by those ignorable false positives. The ability to long-press + multiple select + delete a set of leaks would be a nice feature to have here.

[pyricau]

Note: this should happen after #1202 (which changes the UX by adding groups)

[pyricau]

Closing this:

• Deleting a single leak is tricky as a leak is part of a heap dump which contains multiple leaks. So from a user standpoint there's no clarity around what "deleting a leak" might mean.
• The new organization means leaks are grouped so we don't have all these entries, so the need to delete individual leaks is probably not as high..