Merge menu items for "edit operand" and "edit data"

The instruction operand editor and data operand editor are very
different, but there's no need to impose that distinction on the
user.  They want to edit the operand either way.  We now provide a
single "edit operand" menu item, and open the appropriate dialog
based on what they have selected.

This uses Ctrl+O as the keyboard shortcut, stealing it from
File > Open.

(issue #11)

SourceGen/RuntimeData/SystemDefs.json

@@ -140,7 +140,7 @@
       "Name" : "Atari Lynx",
       "GroupName" : "Atari",
       "Cpu" : "65SC02",
-      "Speed" : "4.00",
+      "Speed" : "4",
       "Description" : "Atari Lynx [placeholder].",
       "SymbolFiles" : [
       ],