Fixes typo.
• Tidies SC.Drag.
• Fixes an issue where SC.Drag would call `dragStarted` on all drop targets regardless of whether they were visible or not, such that a drop target that implemented `dragStarted` may throw an exception because it expects to only be called when visible.
Improves nested store handling of certain circumstances.

SC.NestedStore#lockOnRead, which defaults to YES, is much too aggressive, locking even on attempts to read non-loaded records. This causes problems if, for example, a nested-store record's not-yet-loaded toOne relationship is read (e.g. by being bound to a view), locking it and triggering a load in the parent store. When the load returns, the locked record triggers a full nested-store reset (failing meanwhile to notify the nested-store record instance that it has loaded), or errors out if anything, anywhere in the nested store has changed.

This commit avoids locking nested-store records when read in EMPTY or BUSY_LOADING states. It also modifies the behavior of SC.Store#_notifyRecordPropertyChange, which is where chained store errors are thrown, to switch solely based on the chained record's editable state, and to avoid costly and blunt full-store resets in any case. This allows the parent store to correctly update nested stores when untouched and unlocked records are updated.
Manually unlocks nested store records when committed to the parent store.