Clean up cancel fling gesture and latency info for gestures

This is similar to how the aura code changed with the new snapshot.

Change-Id: I57ef8a8a42c6b880988068c0de26df59fcf436fa
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Do not assume that the webview only contains RWHV delegates

This fixes an assertion for QML code like:

WebEngineView {
    ...
    MouseArea {
       ...
    }
}

Change-Id: I7a9bfb801c81ea53c1cc830b44fe224a0b78db2d
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>