Add getCurrentOpenedExpandedDepth to the table api

[RichardHpa]

There is already a table.getExpandedDepth() function in api but if you multiple nested expanded rows and they are all open (lets then say the getExpandedDepth() = 2). If you close the parent of that nested row then getExpandedDepth will still be 2 even though they arent visible right now.

What would be great is to also have a function that returns the max depth for the currently expanded rows. So in my example above. if I close the parent. getExpandedDepth would still be 2. but getCurrentOpenedExpandedDepth would then be 0.

naming can be different its just the first thing I have come up with