Allow tuning delegated arguments (i.e. exclude some, change defaulting mode...)

[hkoba]

In yatt widget declaration, an argument can be declared as a delegate type like following:

<!yatt:widget foo
    list=[delegate:mylist]
    label=[delegate]  
>

Above means a declaration of new widget foo, which has two delegate type arguments list and label. With these delegate type, the widget 'foo' will get

1. All arguments from another widgets mylist and label (except previously declared same name arguments, which are just ignored).
2. pseudo widget list and label becomes usable in definition scope of this 'foo' widget. When mylist is called, all arguments imported to the widget 'foo' (except explicitly specified in argument list of 'list' call) are automatically passed through to the real call of mylist too. (Same rule applies to label too.).

This automatic argument passing is quite useful to reduce copying argument declaration list from widget to widget. But there was one annoying conflict with another yatt feature: argument defaulting mode !(means required). For example, imagine above mylist is declared with required argument id like following:

<!yatt:widget mylist id=! file style>

In this case, argument 'id' becomes required argument of 'foo' too, even if you explicitly give
the argument 'id' to the call of pseudo widget list. This is quite messy.

So, I decided to add two more features for delegate type.

1. If delegate type declaration has an argument list (like [delegate foo bar]), each specified spec overrides original definition of same name argument.
2. If given name in delegate argument list starts with hyphen (like [delegate -foo]), specified argument is not imported at all.

With this new feature, you can rewrite our problem like following:

<!yatt:widget foo
    list=[delegate:mylist  -id]

You can change defaulting mode/default value too like following:

<!yatt:widget foo
    list=[delegate:mylist  id="?foo"]