ui/crumbs/runningbuild: add optional readonly mode, default off

ui/crumbs/runningbuild: add optional readonly mode, default off

In b947e7aa405966262c0614cae02e7978ec637095 Bob started to introduce code
for a right-click menu, whilst most of the code is non-invasive it does
enable the editable property of the gtk.TreeView which can be confusing.

This change adds a readonly parameter, defaulting to False, to the
RunningBuildTreeView which if True will prevent the editable property from
being set.

CC: Bob Foerster <robert@erafx.com>
(Bitbake rev: d1d6bfab1789d8c0a4faa109bcc06b3cefe62620)

Signed-off-by: Joshua Lock <josh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>