e-wm: upgrade to 0.18.8

Changes since 0.18.7:
* screenshot now displays a helpful error message when it fails
* efm no longer sometimes shows files from other directories
* efm no longer sometimes shows blank filenames
* fix syscon button signals
* fix winlist scrolling
* fix systray appindicator deletion
* fix systray appindicator icon loading

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>