Dispatch an event when an image is resized

[leofeyer]

This PR adds the contao.image.resize_image event, which is dispatched when an image is resized (Resizer::executeResize).

I have chosen resize_image instead of just resize so the class is called ResizeImageEvent instead of ResizeEvent and the constant is called ContaoImageEvents::RESIZE_IMAGE instead of ContaoImageEvents::RESIZE. I can change this though if you want.

[coveralls]

Coverage decreased (-0.7%) to 94.541% when pulling 70d20e79f29e35477e9e3440972d7bee375ac24d on feature/event-listener into 21636e8 on master.

[ausi]

I think we should allow null here too, so that an event listener can remove a resized image that a previous event listener added.

[ausi]

LGTM. I added some comments and I think we should add "symfony/event-dispatcher": "~2.8|~3.0" to composer.json.

[ausi]

How about making the event dispatcher optional and set it via a setEventDispatcher method?

[leofeyer]

I have updated everything according to your comments. Also, I have moved the event dispatcher routines to the top of the executeResize() method, because the resized image might have a different $path and there is no need to mkdir() a directory which is not used later on.

[coveralls]

Coverage decreased (-0.9%) to 94.372% when pulling eb196f3 on feature/event-listener into 21636e8 on master.

[ausi]

Shouldn’t it go into "require"? We are using the interface which could change in a major version.

[leofeyer]

But the event dispatcher is now optional, isn't it?

[ausi]

Yes, but we have to ensure the correct version of the interface we are using. The library might be incompatible with EventDispatcherInterface version 4.0.

[coveralls]

Coverage decreased (-0.9%) to 94.372% when pulling eb196f3 on feature/event-listener into 21636e8 on master.

[ausi]

Merged in 5410142