git-gui: Define a simple class/method system

As most of the git-gui interface is based upon "meta-widgets"
that need to carry around a good deal of state (e.g. console
windows, browser windows, blame viewer) we have a good deal
of messy code that tries to store this meta-widget state in
global arrays, where keys into the array are formed from a
union of a unique "object instance id" and the field name.

This is a simple class system for Tcl that allows us to
hide much of that mess by making Tcl do what it does best;
process strings to manipulate its own code during startup.

Each object instance is placed into its own namespace.  The
namespace is created when the object instance is created and
the namespace is destroyed when the object instance is removed
from the system.  Within that namespace we place variables for
each field within the class; these variables can themselves be
scalar values or full-blown Tcl arrays.

A simple class might be defined as:

  class map {
    field data
    field size 0

    constructor {} {
      return $this
    }
    method set {name value} {
      set data($name) $value
      incr size
    }
    method size {} {
      return $size
    } ifdeleted { return 0 }
  }

All fields must be declared before any constructors or methods.  This
allows our class to generate a list of the fields so it can properly
alter the definition of the constructor and method bodies prior to
passing them off to Tcl for definition with proc. A field may optionally
be given a default/initial value.  This can only be done for non-array
type fields.

Constructors are given full access to all fields of the class, so they
can initialize the data values.  The default values of fields (if any)
are set before the constructor runs, and the implicit local variable
$this is initialized to the instance identifier.

Methods are given access to fields they actually use in their body.
Every method has an implicit "this" argument inserted as its first
parameter; callers of methods must be sure they supply this value.

Some basic optimization tricks are performed (but not much).  We
try to only upvar (locally bind) fields that are accessed within a
method, but we err on the side of caution and may upvar more than
we need to.  If a variable is accessed only once within a method
and that access is by $foo (read) we avoid the upvar and instead
use [set foo] to obtain the value.  This is slightly faster as Tcl
does not need to lookup the variable twice.

We also offer some small syntatic sugar for interacting with Tk and
the fileevent callback system in Tcl.  If a field (say "foo") is used
as "@foo" we insert instead the true global variable name of that
variable into the body of the constructor or method.  This allows easy
binding to Tk textvariable options, e.g.:

  label $w.title -textvariable @title

Proper namespace callbacks can also be setup with the special cb proc
that is defined in each namespace.  [cb _foo a] will invoke the method
_foo in the current namespace, passing it $this as the first (implied)
parameter and a as the second parameter.  This makes it very simple to
connect an object instance to a -command option for a Tk widget or to
a fileevent readable or writable for a file channel.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

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