Added dismissible alert plugin

This is the cleaned up version of https://github.com/wet-boew/wet-boew/pull/5932.
Please note the build will fail because of this issue: https://github.com/wet-boew/wet-boew/issues/5927
What seems to work for me is putting class="h3" on the summary element when an h3 element is used (h4 for h4, etc.). Also seems that the heading needs display: inline so that the pointer and the heading stay on the same line (something that should probably be global to summary/heading combinations in WET - so fixed in src/base/details/_base.css).