Create Replace feature (to pair with Find feature)

[PropGit]

Implement Replace feature (complement of Find feature).

• Request that Find/Replace occupy a compact space on the upper-right area, similar to Google Doc's Find implementation. Find field, followed by Replace field; each about 16 characters in width.
  
• Add F4 key to activate Replace operation on current found selection; does nothing if Find operation not yet performed or no match found
  • Once a find is done, there may be multiple selections and one active found selection. Pressing F4 at this point, if a replace option was filled in, causes the current active found selection to be replaced with the replace text and then the selection becomes unselected (if it doesn't match the find text).