Fix &==: lhs's raw value was rhs's

Copy/paste error.
rulesData: Add drag/drop marker.
selectorController/Source: Rename `source` property to `rule`

While the source of the rule is the rule, it doesn't really make much
sense to be calling it source.
selectorSource: Stub out drag/drop methods.
selectorSource: Implement tableView:pasteboardWriterForRow:

Same deal as the rule data: store the row and the actual data to be
pasted.
selectorsSource: Implement drop validation.

More or less a copy of the behavior in the old rules data source since
it only needs to handle one type.
selectorsSource: implement acceptDrop.
selectorController: register table to accept selector drops.
Update project file with PasteAux.swift
aux: Add no-indices check for withoutIndices
rulesData: Break tableView:acceptDrop:... into type-specific methods.

Sort of necessary to help thinking about the drop types and the
operations needed for each.

Selector pastes can only be dropped on rules, rule pastes can only be
dropped above rules (i.e., you can't merge rules).

Mixed-type pastes are supposed to be impossible, but there's a check
for that anyway.
acceptRuleDrop: let the pasteboard handle UTIs.

No need to have a function for specifically filtering these based on
their UTI.
rulesData: Remove unused function objectIsRulePaste.
rulesData: mark pasteItemToRulePropertyList as internal.

Should be usable from other sources if needed.
rulesData: Rename indices to indicesToRemove.

Just to keep the intent clear.
rulesData: Check if items array is empty. If it is, return early.
rulesData: implement acceptSelectorDrop.

Need to handle when the rule that receives it is also the selected
rule, since the selectors view will need to be reloaded for it.
rulesController: register table to accept selector drops, too.
selectorController: doc comment for the data source.
selectorController: reload table data if selected rule selectors change.

Uses KVO, which I avoided so far, but this is as good a use as any.

Had a hell of a time getting that closure to compile though.. I think
the optional unwrapping might cause it to try to return something like
Void? (i.e., optional void).