Add display method to pst-class

Previously there is no way to distinguish pop-to-buffer and
display-buffer from the perspective method.  This change introduce new
display perspective method to solve the problem.  Note that as the
base class calls the popup method from the display method, there is no
functional change in the defined perspective classes.
Use display method in the two perspective
Cosmetic changes on the two methods
Protect against display method chaining window
Simplify e2wm:dp-two-display
Merge pull request #39 from tkf/display-method

e2wm:$pst-class に display method を追加