Finer zoom control and Vertical Spacing

The zoom in/out is now much finer, and there is now vertical spacing
between slides as well as above and below when selected.
Made slides center in presentation mode.
Made slides center in presentation mode.
Merge branch 'master' into addpoppler
Made zoom control fine
Added some more thread locking for stability.
Merge branch 'master' into addpoppler
Merge branch 'addpoppler' into glib_new