This prevents the column from collapsing on top of one another if the
screen width is too small. This effectively keeps the 2 column layout at
all screen sizes.

The web browsers are quite tolerant to this but not the (x)html
validators. It is necessary to fix this for e.g. automatic web tests
not to fail.

* the contentCol carring layout information (class="col-dm-X")
  is now div#main-center instead of div#pageContent (see cset 1545a8e37c6c)

* HideLeftBar is renamed into HideAsidesBar

Follow up to commit 1c3434c6afd7.