mdPanel: allow callback in offset functions

[crisbeto]

The withOffsetX and withOffsetY methods currently require the offset to be known ahead of time, however this doesn't work too well if the offset is based on the panel itself. This can be worked around by setting the offset in the open callback, however it can cause the panel to jump on slower browsers (IE and Edge). The methods should support passing in a function that allows for the offset to be determined dynamically. Example:

mdPanelPosition.withOffsetX(function(panelRef) {
  return dynamicallyDeterminedOffset;
});

Referencing #9564.