Add manual zoom controls to LNT graphs

The mouse wheel zooming can be cumbersome on some platforms. This change provides good ol' fashioned zoom buttons on the LNT graph, for when mouse wheel zoom does not work.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@208391 91177308-0d34-0410-b5e6-96231b3b80d8