trim
Removes all empty outer rows and columns e.g. all rows and columns counted from left, right, top and bottom that have no views in their cells.
That means after calling the first and last row as well as the first and last column have at least one component in their cells.
Attributes rows and columns get updated according to new dimensions.
If the grid was empty the grid gets trimmed to size 0x0.
Return
true if the grid has changed by this operation, false otherwise.