IDL compiler: [Custom] and [CustomGetter] getters (+ fix Perl [Custom] readonly)

This also fixes a bug in the Perl CG:
For [Custom] readonly attributes: the Perl CG generates
a setter (*AttributeSetterCustom) in the header, which is incorrect,
since readonly attributes can't be set.

Compare for instance:
Source/core/events/Event.idl
[Custom] readonly attribute Clipboard clipboardData;

Source/bindings/v8/custom/V8EventCustom.cpp
...which doesn't implement the setter.

Also consistency changes in IDLs:
[Custom] => [CustomGetter] for read only (and not [Replaceable]) attributes
[CustomGetter, CustomSetter] => [Custom]

Doc:
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-Custom-m-a-CustomGetter-a-CustomSetter-a-

BUG=239771

Review URL: https://codereview.chromium.org/26420002

git-svn-id: svn://svn.chromium.org/blink/trunk@159109 bbb929c8-8fbe-4397-9dbb-9b2b20218538