Custom pan recognizer + loading widget

[vixez]

Custom pan recognizer and detector

In some cases there are multiple GestureRecognizers in a certain area. This could result in the corner dots becoming unresponsive. For example, I have a tab bar and multiple horizontally sliding pages. This would cause the HorizontalDragGestureRecognizer always winning in the arena. Because of that you could not drag the corners horizontally, only vertically. This PR fixes that issue.

Custom loading widget

Apps often use their own loading overlay, so it is now possible to replace the default CircularProgressIndicator