State: Prevent unnecessary state updates to edit-post preferences

[aduth]

Related: #5974 (comment)

This pull request seeks to resolve two issues with unnecessary state updates and dispatching to the edit post store:

1. Most preferences values were not comparing to the current state, merging into the state object and creating a new reference even if the new preference was the same. By returning a new object reference, all state subscribers were being invoked. With these changes, leveraging combineReducers has the benefit of avoiding state updates when each key of preferences is strictly equal.
2. The store subscriptions introduced in Opens the block inspector automatically when the block is selected #5882 were dispatching in response to a change in the UID of the selected block, when it was sufficient to listen for a change in plainly whether or not there is a selected block (only having two states true and false, rather than unlimited states of possible set of block UIDs). With these changes, the subscribe listener will only dispatch when going from a state of cleared selection to having a selection, but not when changing selection from one block to another.

Testing instructions:

Verify that there are no regressions in the behaviors of preferences (toggled panels, active sidebar, editor mode).

Repeat testing instructions from #5882, verifying no regressions.

[youknowriad]

Nice refactoring 👍

[youknowriad]

LGTM 👍