constraints: account for decorations when positioning modal dialogs

What we want to achieve is that the dialog is visually centered
on the parent, including the decorations for both, and making sure
that CSD/frame_extents are respected.

https://bugzilla.gnome.org/show_bug.cgi?id=707194
Use utility functions to convert between frame and client rectangles

There are extensive places in the code where we convert between the client
rectangle and the frame rectangle. Instead of manually doing it use
new helper functions on MetaWindow and the existing meta_window_get_outer_rect().

This fixes a number of bugs where the computation was being done incorrectly,
most of these bugs are with the recently added custom frame extents, but
some relate to invisible borders or even simply to confusion between the
window and frame rectangle.

Switch the placement code to place the frame rectangle rather
than the client window - this simplifies things considerably.

https://bugzilla.gnome.org/show_bug.cgi?id=707194
MetaFrame: Cache borders

Cache the computed border size so we can fetch the border size at
any time without worrying that we'll be spending too much time in
the theme code (in some cases we might allocate a PangoFontDescription
or do other significant work.)

The main effort here is clearing the cache when various bits of window
state change that could potentially affect the computed borders.

https://bugzilla.gnome.org/show_bug.cgi?id=707194
Stop passing around MetaFrameBorders

Instead of passing around MetaFrameBorders, compute it when we need it.
This also allows us to know that we are using MetaFrameBorders only for windows
with frames (where it is meaningful) and not for frameless windows, which
can have custom borders which we need to interpret differently.

https://bugzilla.gnome.org/show_bug.cgi?id=707194
Rename meta_window_get_outer_rect() to meta_window_get_frame_rect()

For clarity, rename meta_window_get_outer_rect() to match terminology
we use elsewhere. The old function is left as a deprecated
compatibility wrapper.