Implements pools

[chipit24]

Related issue: #2. Note: This PR only implements pool functionality; lanes will come in another PR.

• If there are currently elements on the page but no pools, dragging a pool onto the page will move all elements into the new pool.
• Dragging elements around in the pool will expand the pool.
• If there are pools on the page, elements must be dragged onto the pool, otherwise it won't be added to the diagram.

What's currently not working:

• Parsing pools in XML ("Upload XML").
• Dragging elements between pools (currently just expands the pool).

[tdondich]

I cannot link elements inside pools. I tried a start event and a task in a pool and then tried to connect the start to the task.

[tdondich]

After deleting a pool, I can no longer drag items over the canvas.

Steps to reproduce:

• Create a pool
• Drag start event into pool
• Delete start event
• Delete pool
• Attempt to drag control into canvas

[velkymx]

I need to be able to resize the pool to add new elements.

[chipit24]

I need to be able to resize the pool to add new elements.

@velkymx Currently, the pool expands when you drag an element near the edge, but this functionality will be modified (to allow dragging elements between lanes and pools). But I will make it so pools and lanes will still resize if you drag elements near the edge.

Resizing elements sounds like a task that should be handled in a different PR (and possibly different issue), to:

• Cut down on merge conflicts.
• Keep PRs smaller for quicker review and merge.
• More easily track progress.

[tdondich]

@chipit24 appears to be an issue. Reproduction steps:

• Drag a start event
• Drag a pool. Start event properly goes into initial lane of pool
• Create the lane crown element to create a new lane
• click on lane start event is in
• Notice start event is no longer visible