Generic alt and title for Image Sources, no src in source tags, phpstan and no more Neos 4.3 support

Changes;

1. While the source tag allows an src attribute it is ignored in picture tags. Since this is all Kaleidoscope cares about we can remove this safely.

2. The codebase is now tested with phpstan. To do so we had to drop the Neos 4.3 support.

3. All imageSource FusionObjects now support the fusion attributes 'alt' and 'title' that will be used as fallback for the
   respective attributes of the img-tag. That way it is no longer necessary to pass three separate properties around.

Examples:

1. Use title and alternativeText from node properties with fallback to the asset title:

imageSource = Sitegeist.Kaleidoscope:AssetImageSource {
    asset = ${q(node).property('image')}
    title =  ${q(node).property('title') || q(node).property('image').title}
    alt = ${q(node).property('alternativeText')}
}

2. Set alt and title for dummy image source

imageSource = Sitegeist.Kaleidoscope:DummyImageSource {
    alt = 'Alternate assigned to source'
    title = 'Title assigned to source'
}