Make the ModalBarrier not dismissible when barrierEnabled is true

[franciscrispin]

Related issue: #5

The ModalBarrier that is shown when barrierEnabled is true has a dismissible value that defaults to true. This means the ModalBarrier can still be dismissed even though the barrier is enabled.

Approach 1: This PR sets the dismissible value to false so the ModalBarrier cannot be dismissed.

Approach 2:

If it is the intention of the author to leave dismissible as true, another way to solve the issue is to make the following change:

// https://github.com/FlorinMihalache/flutter_progress_hud/blob/master/lib/flutter_progress_hud.dart#L128
IgnorePointer(
   ignoring: widget.barrierEnabled || !_isShow,
   ...

Approach 1 seems preferable because it is slightly counter-intuitive if the ModalBarrier can be dismissed even though barrierEnabled is true.

[franciscrispin]

Hi @FlorinMihalache, could you help review this PR, thanks!

[glintpursuit]

Please merge this PR

[FlorinMihalache]

I know it's been some time, but I merged your PR. Thank you @franciscrispin :)