馃崚 applets/taskmanager: Fix grid layout for forced multiple rows/columns
In the case of forced multiple rows/columns, to calculate the effective width and height of the TaskList we need to know the exact number of rows and columns, and also the average occupation of those (fractional if some rows are not full)
BUG: 488702 FIXED-IN: 6.1.1
(cherry picked from commit a1710e8f)