Fix several bugs relating to ToolStrips with Flow layout

Fix several bugs relating to ToolStrips with Flow layout

Specifically:
  - Do not take into account unavailable ToolStripItems.
  - Use preferred size of ToolStripItems instead of current size.
  - Take into account the ToolStrip's padding.
  - Do not fill the ToolStrip height during layout, which is wrong
    for multi-row ToolStrips.  This reverts commit f124ea39, which
    fixed Novell bug #469196.  As an alternative fix for that issue,
    set ToolStripSeparator's Dock to Fill in its constructor.