Add hint to "tags" input

[CodeWringer]

The list of known tags should be displayed somehow. Two possible approaches:

1. An info-bubble displays the list of known tags (including system- and user-defined).
2. Once the input receives focus, a list of the known properties pops up. This would require writing a new component that supports:
   1. A list that pops up beneath or above the input field, depending on how much space there is available.
   2. Selection of items from the list (via mouse and keyboard).
   3. Narrowing the list of results by typing in the input field.