Show a list of all packages being built

[tusooa]

First of all, thank you for this wonderful tool and the EFI guide. (>w<)

This PR makes showem display a list of packages being built now.
Failed builds will not be on the overview list, but their logs are still tracked, and can still be displayed provided it is the newest log among all (i.e. all other builds have terminated).
It also solves #2, as it takes the individual environment into consideration.

One behaviour change is included in this PR.
On master, showem will track all builds in the default PORTAGE_TMPDIR.
Now it will only track builds that terminates after showem starts. In other words, if a build failed before showem starts, it will not be tracked by showem even if its build directory exists under the default PORTAGE_TMPDIR.

[sakaki-]

31 Oct 2020: sadly, due to legal obligations arising from a recent change in my 'real world' job, I must announce I am standing down as maintainer of this project with immediate effect. For the meantime, I will leave the repo up (for historical interest, and it may be of use to others); however, I plan no further updates, nor will I be accepting / actioning further pull requests or bug reports from this point. Email requests for support will also have to be politely declined, so, please treat this as an effective EOL notice.

For further details, please see my post here.

With sincere apologies, sakaki ><