Work In Progress: CPU behavior, UI changes

This is something of a "Work in Progress" checkpoint of several features
that are all half baked:

1. Allow loading of 16KB ROM files at address $C000 at run-time, not
   just at startup. See the "Load ROM..." File menu item.

2. Introduces the notion of "CPU Behaviors", so the core 6502 CPU
   implementation can match the behavior of either an early NMOS 6502, late
   NMOS 6502, or CMOS 65C02. Very little of this is actually implemented so
   far.

3. Adds a completely bogus implementation of the 6522 VIA (it
   does absolutely nothing right now).

4. Changes the address of the ACIA in the simulated system to match a
   real hardware implementation I put together.

samples/echo.asm

@@ -3,7 +3,7 @@
 ;;
 
 
-.alias iobase   $c000
+.alias iobase   $8000
 .alias iostatus [iobase + 1]
 .alias iocmd    [iobase + 2]
 .alias ioctrl   [iobase + 3]

samples/ehbasic/basic.asm

@@ -441,7 +441,7 @@ Ram_top		= $C000	; end of user RAM+1 (set as needed, should be page aligned)
 
 ; This start can be changed to suit your system
 
-	.org	$D000
+	.org	$C000
 
 ; BASIC cold start entry point
 

samples/ehbasic/min_mon.asm

@@ -15,7 +15,7 @@ NMI_vec	= IRQ_vec+$0A	; NMI code vector
 
 ; setup for the 6502 simulator environment
 
-IO_AREA = $C000
+IO_AREA = $8800
 ACIAdata	= IO_AREA		; simulated ACIA r/w port
 ACIAstatus  = IO_AREA+1
 ACIAcommand = IO_AREA+2

samples/ehbasic/symon.config

@@ -1,6 +1,6 @@
 MEMORY {
-RAM1: start = $0000, size = $C000;
-ROM1: start = $D000, size = $2F00, fill = yes;
+RAM1: start = $0000, size = $8000;
+ROM1: start = $C000, size = $3F00, fill = yes;
 MONITOR: start = $FF00, size = $FA, fill = yes;
 ROMV: start = $FFFA, size = $6, file = %O, fill = yes;
 }

samples/hello.asm

@@ -3,7 +3,7 @@
 ;;
 
 
-.alias iobase   $c000
+.alias iobase   $8000
 .alias iostatus [iobase + 1]
 .alias iocmd    [iobase + 2]
 .alias ioctrl   [iobase + 3]