Attach an image to a node by pasting the image

[mmahmoudian]

This is continuation of a previous feature request (#173). At the moment (v3.0.0) user can paste the image and consequently a node will be created with empty text and the pasted picture is attached to the node. Considering that the functionality is there already, it would be nice if a user can paste a photo and it automatically gets attached to the node when:

1. the text box of the node is in focus (even while typing something
2. when a node or nodes is selected (e.g via Shift+click)

This behavior would also be consistent with the current behavior that a user can multi-select nodes and change their colors or move them together.