* center the line in the layout category headers in the middle of the lowercase...

* center the line in the layout category headers in the middle of the lowercase chars
    * some messages for the statusbar
    * configuration option for grouping of the layouts
    * put category headers above the first item of each category in the
      layout list.
      To avoid scrolling we have to sneak in space for the headers.
      So we tweak this value accordingly. It's not nice, but the
      only possible way it seems.

    Note 1: Because QComboBox uses the "first row's height * rowCount" to
    estimate the height of the popup, we have to fake some extra pixel
    lines to avoid scrolling because of the extra space taken by the
    headers. This is a hack, but I don't see a better way to establish that.

    Note 2: An alternative implementation is to insert headers as real list
    items. But it seems that QListView cannot skip them in cursor
    navigation then. So we stick to the upper approach.

    * remove checkbox in layout menu

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@23626 a592a061-630c-0410-9148-cb99ea01b6c8