Load mongo table data in chunks

This changes the main table view to load data from the
Mongo database in small chunks rather than all at once.

Now, when MongoChem starts only 250 rows of the database
are loaded. When the user scrolls the table to the bottom
additional rows will be requested automatically.

Change-Id: Ica7a8365fb38788b72b705d51fbb94e7552bcba2