fix(ion-backdrop): new ion-backdrop can prevent background scrolling

[manucorporat]

Short description of what this resolves:

New ion-backdrop component can prevent background scrolling fixing: #6656 in Alerts, Popover, actionsheet, loading...

Changes proposed in this pull request:

• Replace all <div class="backdrop" ...> with <ion-backdrop>

• MenuBackdrop is not longed needed

• We don't longer have to listen for touchmove events!! performant solution

• Backdrop injects a class at body level in order to prevent scrolling:

  body.stop-scroll scroll-content {
  overflow-y: hidden;
  overflow-x: hidden;
  }

Bonus: this code was repeated all over the place, now it is much more compact.

Before:

<div (click)="bdClick()" tappable disable-activated class="backdrop" role="presentation"></div>

After:

<ion-backdrop (click)="bdClick()"></ion-backdrop>

Ionic Version: 2.x

Fixes: #6656

[adamdbradley]

@manucorporat How does this perform on slow Android during a transition? My concern is modifying the body element, which causes a repaint on the entire app, so it may occasionally add a flicker. Overall I think it's a great solution, just want to double check first that slow Android doesn't have a flicker.

[manucorporat]

@adamdbradley on device iOS even in a slow UIWebView I didn't notice any flicker, on android I didn't tried... BUT! if this is a iOS only issue, we may don't have to modify the body on android... right?

I can do some quick research today.