Provide full set of editing operations

[kpreid]

Needed editing operations on a blockset:

• Delete a block type. Done in commit 00dd645caba6623515a2f7aa397afdab50b1ffbe
• Create a new block type (probably by cloning an existing one). Done (barely)
• Reorder block types.

Needed editing operations on a block type:

• Set solidity flag. Done in commit 7484b3dcd6082de8a37130b05bff9cb053339442
• Set circuit behavior. Done in commit 7484b3dcd6082de8a37130b05bff9cb053339442
• Add/remove/replace world, assign color.

Needed editing operations on a world:

• Change the size, by adding/removing margins on all six sides.
• Perform global find/replace on block IDs to adapt to blockset changes. (Need for this would be mitigated by switching to named block IDs, but see next item.)
• Perform local (defined by a selection region) find/replace on block IDs for easy editing of block designs.
• Change the blockset in use (?).
• Place the first block in a completely empty world.
• Set the subdata of a block. (Or should this be strictly the responsibility of block circuits?) Done in commit 7e2c374ffe6c73d027c6fb043652d842bbb156b7
• Copy sections of a world to another world. (Besides terrain/building editing, this is also relevant for block designs and circuits.)

Needed “editing” operations for all persistent objects:

• Rename. (Can currently be done as delete+save, but that is unintuitive. Also needs to be made safe against references from other objects.)
• Check for usage by other objects.
• Export
  • for objects other than worlds. Done in commit 9a5fc7c743f8ee03e5de9c8d6b549c9451a722a4.
  • with references or as complete subgraph.
• Import for objects other than worlds. Done in commit 86c9c61e1b1e3f6705ae342af8df86605777e84f.