Replace AbstractClient::adjustedSize() with a simpler alternative

Currently, adjustedSize() does two things - it computes and constrains
the natural size of the window.

In many places where adjustedSize() is used, the natural size doesn't
need to be constrained and in some it's actually undesired because
AbstractClient::constrainClientSize() doesn't allow the client size to
be 0x0, which can happen when dealing with a client that has no buffer
committed yet.

This change replaces adjustedSize() with implicitSize(), which simply
calculates the natural frame size based on the current client size.

If the frame size needs to be constrained, for example during
interactive move-resize, use constrainFrameSize() or if you need to
constrain the client size - constrainClientSize().