ClearRow functionality added

Added a numpad1 debug key in gameworld.cs to check if it works
IsRowFull/Empty and PopBottomRow

Check method if a row is empty or not, popbottomrow to remove bottom row
and move everything down.
Cleaned up grid methods, updated debug keys.

Numpad 1 = clear a random row
Numpad 2 = new function: check for empty rows and if you find one pop
clean it up, so pop it.
Merge pull request #1 from TijmenUU/tetrisgridManagement

Tetrisgrid management