Setting enabled to false causes transparency to stack

### Problem description

When setting enabled to _false_ on a `Composite` containing a `TextView`, the TextView is grayed out. Setting enabled to _false_ on the `TextView` as well causes the disabled visualization to stack, making it "even more disabled".

### Expected behavior

Setting enabled to _false_ state should be consistent no matter how often it is applied in the layout hierarchy. The initial disabled state is also rather faint making it not directly obvious that the widget is disabled.

### Environment

- Tabris.js version: 2.0.0
- Device: iPad Pro 9.5
- OS: 10.3.3

### Code snippet

Both `TextView`s should have the same visual disabled appearance.

```javascript
const {TextView, Composite, ui} = require('tabris');

let composite = new Composite({
left: 0, right: 0, top: 0, bottom: 0,
enabled: false
}).appendTo(ui.contentView);

new TextView({
left: 16, top: 16, right: 16,
text: 'TextView',
}).appendTo(composite);

new TextView({
left: 16, top: 'prev() 16', right: 16,
text: 'TextView',
enabled: false
}).appendTo(composite);
```