refactored window handling

the "listwindow" and the "previewwindow" are now handled
in a more intelligent way: instead of re-creating a new
buffer and its content for each of both on each (re-)open,
now a buffer for the content of both windows is just created
one time and then only closed and re-opened in new split windows.

further from now on option "nomodifiable" is used to ensure
that the user cannot change content of "listwindow" or "previewwindow"
content.
implemented usage of status line

both "listwindow" and "previewwindows" now have
their individual statusline
repaired RefreshBuildTargetList feature

when the user presses 'R' while being in the "listwindow",
then './waf list' is executed again and the "listwindow"
is built again from scratch. this feature already existed,
but did not work anymore because of changes in previous
refactoring commits. this commit should fix it.