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.
Merge pull request #42 from tkf/dont-set-killed-buffer

Don't set killed buffer