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
Add after-bury perspective class method
Cosmetic changes on base perspective class
Use after-bury method in code and two perspectives
Cosmetic change on code/two perspectives
Document about arguments for after-bury
Refactor e2wm:dp-two/code-after-bury

Call e2wm:pst-get-wm once and make the code cleaner.
Tweak e2wm:dp-two/code-after-bury

Select main window after burying sub window, rather than going to
some random window.
Dynamically create the blank buffer

Previous version can be problematic when the blank buffer is killed.
This is fixed by adding e2wm:get-blank-buffer.

The configuration variable e2wm:c-blank-buffer is replaced by
e2wm:c-blank-buffer-name.
Check if main buffer is alive in imenu plugin
e2wm:history-get* returns live buffers

Deleted buffers are filtered out by e2wm:history-filter-killed-buffers now.
Protect against display method chaining window
Fix a name error in pst-after-bury logger
Suppress the default behavior when switching to right
Merge pull request #44 from tkf/fix-dp-two-switch-right

Suppress the default behavior when switching to right
Better error message from display-buffer

To check this patch, execute the following code
```lisp
(display-buffer "*no such buffer*")
```
Without this patch, it fails with a suspicious error.  When
this patch is applied, it fails with a clean error message.
Merge pull request #45 from tkf/better-error-in-display-buffer

Better error message from display-buffer
Fix base after-bury when other window showing the buffer