[Scenario] Pad activity tagging combo box

Add padding to give the tag selection combo box
a height similar to that of other combo boxes
in the Info View.
[Scenario] Hide empty tag area in Info View

If there are no tags to display, hide the tag panel
which would be used to display them in the activity's
Info View. Otherwise, this empty panel takes up a
few pixels of vertical space, causing alignment
issues for the label on the left.
[Scenario] Simplify activity tag combo box

Simplify activity tag combo box, creating the
always-there Add a Tag message using the
list cell renderer instead of "tricking" the
combo box with a custom editor. This has the
added benefit of restoring the default focus
color (previously missing)