The "add platform symbol file" and "add extension script" buttons
create a file dialog with the initial directory set to the
RuntimeData directory inside the SourceGen installation directory.
This is great if you're trying to add a file from the platform
definitions, but annoying if you're trying to add it from the
project directory.
It's really convenient to not have to hunt around though, so now
there are two buttons: one for platform, one for project. The
latter is disabled if the project is new and hasn't been saved yet.
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.
