Show placeholder in number of rows dropdown when showing all

- Implemented placeholder in dropdown in getDropdown in PMA_Util
- Show 'All' as placeholder when Show all is checked, in "Number of
rows:" dropdown

Signed-off-by: Nisarg Jhaveri <nisarg.jhaveri@research.iiit.ac.in>
Merge pull request #1412 from nisargjhaveri/show_all_dropdown

Show placeholder in number of rows dropdown when showing all