Bug 70760

'Select Conversation' and 'Subscribe To Public Conversation' need usability
improvements.

My issues with the current UI is the use of two dialogs as well as using
multiple selection in 'Select Conversation'

First issue is mostly because I prefer a management tool as opposed to a tool
which allows you to add certain elements.

The second is because selecting multiple entries using a modifier key is
error-prone, especially with large lists.

I am attaching a proposed alternative for managing subscriptions for conversations.

I believed that the ui I propose provides the same (if not more functionality)
as the existing ui. I still have reservations whether the workflow of the
proposed ui would be intuitive to the users.

I provide a brief description in case the situation is not self-explanatory.

Left pane contains a tree of subsriptions. The user can select conversations and
unsubscribe from them. When this occurs they are added to the 'Pending
unsubsription' node. The user can reset the current changes. A context menu on
the tree leafs allows more control over the reset operation.

Right pane is split into two panels. The top panel provides a filter feature
(equivalent to existing 'Find'). It is enabled by default. If disabled 'Search'
acts like 'Select conversation'.

Whether or not a filter is applied, the search results are shown in the bottom
panel. The user can check which conversations they want to subscribe to. If the
'Subscribe' button is selected, the selected conversations are added to the
'Pending subscription' tree node.