Image Editing: Image Frames

[p0mm]

this PR was originally a response to #4183, i have since split it up into image insertion #4966 and Image Frames (this PR)!

Image Frames

General Idea:

• edit image while it is inside the frame: allow images to be cut!
• allow the user to plan for multiple images before inserting all of them in one go!
  -> make adding multiple images easier to plan
• possibility to template?

Demo:

Progress.webm

Plans:

• define space by dragging the mouse (click - drag - let go)
• have a nice visual to see what space you are choosing
• frame for image comes into existence
• its size can be adjusted
• it will remain if you save, close and open xournalpp again
• allow multiple image frames to coexist
• add image to the frame by clicking it
• give a visual so this mechanic is easily understandable (show pointer as tiny image icon while hovering over image frame)
• add image by dragging and dropping it into the frame
• give different options on whether the image is scaled up, down or stretches and fills the space
• have a default setting for these options available in the preferences
• still keep a simple add image directly option easily available, by having this be a different tool!
• turn image into frame by clicking it with the frame tool!
• cut images down to fit the image frame
• turn image frame into image
• allow image frame to be cloned

[forgottosave]

Awesome feature, would be great to have this! To actually be able to scale the image before inserting it really saves you the troubles of having a giant image covering the whole page.