Set initial focus to appropriate field for project properties

If you double-click a project symbol declaration, the symbol editor
opens.  I found that I was double-clicking on the comment field and
typing with the expectation that the comment would be updated, but
it was actually setting the initial focus to the label field.

With this change the symbol editor will focus the label, value, or
comment field based on which column was double-clicked.

The behavior for Actions > Edit Project Symbol and other paths to the
symbol editor are unchanged.

Also, disabled a wayward assert.

SourceGen/DisasmProject.cs

@@ -1541,7 +1541,10 @@ namespace SourceGen {
                                     sym.SymbolSource == Symbol.Source.Platform) {
                                 DefSymbol defSym = sym as DefSymbol;
                                 int adj = 0;
-                                Debug.Assert(operandOffset < 0);    // outside file scope
+                                // NOTE: operandOffset may be valid, since you are allowed to
+                                // define project symbols for addresses inside the file.  I
+                                // don't think we need to fiddle with that though.
+                                //Debug.Assert(operandOffset < 0);
                                 if (sym.SymbolType != Symbol.Type.Constant &&
                                         attr.OperandAddress >= 0) {
                                     // It's an address operand, so we can compute the offset.