Added a Width column to the list in the project symbol editor.
Changed the local variable table editor and the project symbol editor
to use DataGrid instead of ListView. This gets us easy sorting on
arbitrary columns. The previous code was reloading the display list
after every change; now we just add/edit/remove individual items,
which helps keep the list position and selection stable.
Both dialogs got a couple extra radio buttons for selection of
single character operands. The data operand editor got a combo box
that lets you specify how it scans for viable strings.
Various string scanning methods were made more generic. This got a
little strange with auto-detection of low/high ASCII, but that was
mostly a matter of keeping the previous code around as a special
case.
Made C64 Screen Code DCI strings a thing that works.
It's not quite the same as the character encoding -- sometimes we
want a mix of things -- so it gets its own enum. The value is
saved to the project file, but not actually used yet.
Also, moved some combo box strings into XAML resources.
