allowed the add block panel to be pinned down

Regarding issue #1688

- Clicking on the add block button opens it (but does not lock it)
- Clicking a second times locks it down.
- Clicking a third time closes it

If the panel is not locked (pinned) then no change from previous behavior.

If the panel is pinned, it is reopened after a block had been added (or its add form closed/cancel).

Tell me how you guys feel about it.