Add option to run a window manager

Allow run a window manager for UI tests running using virtual display
feature. If the window manager command specified in the properties file
is found then it will be run before open any browser window. This will
allow the browser window be maximized.
Merge pull request #1261 from elyezer/ui-wm-run

Add option to run a window manager
Handle comments from window manager feature

Handle comments made on pull request #1261, related to windows manager
feature:

* Update information on README
* Update comment on properties file
* Remove line with a print statement
Merge pull request #1262 from elyezer/ui-wm-run

Handle comments from window manager feature
Add EntityReadMixin to some entities

When asked for information about an entity, the API often returns data that is
inconsistently named, inconsistently structured, or incomplete. Thus, the
EntityReadMixin can only be added to a few entities. Do so, and add a test
verifying that the mixin works.