Summary:
Currently, there are a couple of issues with sizeForClientSize(). First
of all, we have a method called clientSizeToFrameSize() which does similar
thing except applying geometry constraints and checking window rules. The
other issue is that sizeForClientSize() is doing a bit too much, it checks
window rules, it applies a bunch of geometry constrains. Sometimes it
does not perform conversion between client sizes and frame sizes!

This change attempts to address those issues by replacing sizeForClientSize
with two similar methods and changing semantics of some methods of the
X11Client class.

The most significant difference between sizeForClientSize() and the new
methods is that neither constrainClientSize() nor constrainFrameSize()
check window rules. This is up to users of those methods. In many places,
we don't have to check window rules because we check isResizable(),
which returns false if the frame size is enforced by a window rule.

Reviewers: #kwin, davide...