Add support for follow-up/sequential actions.

[dnrce]

Migrated from the original issue #185 at https://www.assembla.com/spaces/tracks-tickets/tickets/185

I find that when I create a project, I often come up with several future actions. However, these actions are sequential, i.e. action 3 depends upon the completion of action 2, action 2 on action 1, etc. As such, I only want to see the next action (action 1), and not the follow-up actions (2,3). Looking through previous tickets, I noticed that others have requested a similar feature, though it appears that the current version of Tracks (svn revision 170) does not yet support this.

After (briefly) looking through the code and the database, I thought of one way that this might be implemented. Two new columns could be added to the 'todo' table: 'active' (a boolean which could prevent display) and 'predecessor_id'(or something similar). An existing action would then have a link to create a follow-up action; the follow-up's predecessor_id would be set to that of the existing action, and would be marked as inactive. When the existing action is marked as done, it would look for any follow-up actions and mark them as active (hence one action could have multiple follow-ups).

The active/predecessor_id columns could also be added to projects, so that projects could be treated in the same fashion. One issue here, though, would be determining if a predecessor_id refers to an action or a project.

A side benefit of adding an active flag to actions/projects is that it could also be used to for the Someday/Maybe context; any actions/projects added to Someday/Maybe could be marked as inactive so that they are not shown in the 'Home' display.