Xournalpp: Paper background image pattern

[gdolle]

Is your feature request related to a problem? Please describe.

Currently, it's possible to import a background image, but the canvas will fit the size of this image.
It would be nice to apply this image as a repeating pattern for a given paper format.
This issue is related to #2359.

Describe the solution you'd like

Add a menu entry Journal -> Paper background -> Image pattern
This image is set at set at (0,0) repeated along x,y.
Maybe provide also several pattern with the software.

Additional context
An example of usage of this feature would be to get a nice background blackboard/whiteboard for live courses (similar to revealjs chalkboard, see pattern1/pattern2 and the chalkboard demo demo by clicking second icon bottom left)

[gdolle]

With this feature it would also be nice to add the possibility to overlap 2 patterns, for example one for the background image, and one for the rules/grid/... pattern image.

[robinp7720]

I think this would work very well with #2359. It suggests an ability to create custom page templates/backgrounds from within xournalpp and I'm personally a fan of being able to define page templates with custom background images.

[noah1510]

Should I add this to the list of thing that should be done in #2359 ?
That way background related problems are in one place?