Extend support for placeholders

[kamilburda]

To support use cases like "set layer offsets to 50% of image size", we need to provide additional placeholders, particularly for integer and float parameters. This way, the user has the option to specify a value which will be dynamically derived from the attributes of an object, e.g. image or layer width/height.

Conversely, one may want to apply a procedure on a specific layer or image rather than the currently processed layer/image.

In the GUI, we need to allow the user to toggle between the regular and the placeholder widgets when editing procedures/constraints.