Add the 'non-overlapping' feature to `alert.defaultStyle.atScreenEdge` = 1/2

[dayfuaim]

Now alerts' atScreenEdge behave the following way:

atScreenEdge   - 0: screen center (default); 1: top edge; 2: bottom edge . Note when atScreenEdge>0, the latest alert will overlay above the previous ones if multiple alerts visible on same edge; and when atScreenEdge=0, latest alert will show below previous visible ones without overlap.

I think more logical (if I could say that) would be:

• non-overlapping top down, when 1
• non-overlapping bottom up, when 2

Please, make at least a key like overlap (0/1, default is 1) for that.

Thank you in advance. :)