Update build steps related panels

 * Merge BuildSettingsWidget and BuildSettingsSubWidgets. I don't
   see much need for this separation and it makes the indention harder
   to do.
 * Override contentsmargins set by the ProjectsWindow.
   This enables the toolwidget to be displayed properly.
 * Add a remove button to the toolwidget of the buildsteps. Remove the
   global one.
 * Simplify code a bit by using QSignalMapper instead of mapping signals
   manually.

Reviewed-by: dt