This new class will open and parse files (either images directly or .pp
project files) and one can get the parsed Image as well as other
information from it.
This patch removes a bunch of 'try_create_from..." methods from Image in
favor of using the ProjectLoader.
The only json_metadata that is available are Guides for now.
This patch adds the logic for the GuideTool. Pulling in from outside the
image creates a new Guide, moving a Guide outside of the image deletes
it and a Guide can be deleteted via the context menu.
A Guide is considered clicked when the cursor is less than 20 pixels
away from the line.
The Mask class represents an opacity mask over a rectangular section
of an image, linking every pixel to an alpha value ranging from 0 (not
selected) to 255 (fully selected). "Partially selected" pixels can be
used to simulate anti-aliased curves.
This class will be used as the basis for the new non-rectangular
selection feature.
