GtdTags to search for active tags

It would be useful to develop :GtdTags command to:

• look for the results of a formula search
• answer with the list of tags found in that result
• display this answer in a buffer where we can search for details

Example:

We want to search for waiting topics.

:GtdTags !waiting

Lets say it returns:

• #project1
• #personX
• #personY

Then we have that kind of display, ready to launch detailled searches by hiting <Enter> key:

#personY (5 results)
#projects1 (3 results)
#personX (1 result)

The display is ordered by descending quantity of results. The searches have to be done in the context of the initial query (here, "!waiting").