SVGHMI: add "unsubscribable" property to widgets in order to generalize what already happens for jump buttons.
In most cases jump buttons do not really subscribe to pointed HMI variable, path is given as a relative page jump path. When widget.unsubscribable is set to true, no subscription is made on page switch, but still offset is updated.
This fixes bug happening on relative jump buttons without "disabled" element where offset did not change on relative page switch.

#
# Dockerfile for wxPython3.0 running on python2.7
#

FROM python:2.7-stretch

RUN set -xe \
    && apt-get update \
    && apt-get install -y --no-install-recommends python-wxgtk3.0 python-matplotlib \
    && apt-get install -y --no-install-recommends python-xvfbwrapper xvfb \
    && apt-get clean