More improvements in TMemStat::Show

//
// TMemStat::Show creates 3 canvases.
// -In canvas1 it displays a dynamic histogram showing for pages (10 kbytes by default)
//  the percentage of the page used.
//  A summary pave shows the total memory still in use when the TMemStat object
//  goes out of scope and the average occupancy of the pages.
//  The average occupancy gives a good indication of the memory fragmentation.
//
// -In canvas2 it displays the histogram of memory leaks in decreasing order.
//  when moving the mouse on this canvas, a tooltip shows the backtrace for the leak
//  in the bin below the mouse.
//
// -In canvas3 it displays the histogram of the nbigleaks largest leaks (default is 20)
//    for each leak, the number of allocs and average alloc size is shown.
//
//
// Simply do:
//   root > TMemStat::Show()
// or specifying arguments
//   root > TMemStat::Show(0.01,20,"mydir/mymemstat.root");
//
// The first argument to Show is the percentage of the time of the original job
// that produced the file after which the display is updated. By default update=0.01,
// ie 100 time intervals will be shown.
// The second argument is nbigleaks.
// The third argument is the imput file name (result of TMemStat).
// If this argument is omitted, the script will take the most recent file
// generated by TMemStat.


git-svn-id: http://root.cern.ch/svn/root/trunk@36362 27541ba8-7e3a-0410-8455-c3a389f83636