Idea: flexible background ruler descriptions

[letsfindaway]

Currently OpenBoard has a set of built-in ruled backgrounds: lines, checked and Seyes. But I can imagine that there would be much more patterns which can be useful: Lines for music, other countries primary school patterns, technical drawings, etc.

The idea is to create a flexible description of such patterns.

• Avoid adding features specific for a limited target audience. Instead be extensible.
• Create a generic ruler description based on repeating lines in a specified angle.
• Provide a standard set of ruler descriptions, probably for lined and squared only.
• Probably provide a set of other ruler descriptions, e.g. for music, technical drawing, or other countries' primary schools.
• Easy way for the user to load an alternative ruler description, e.g. by dropping a pattern file on the background palette.

How flexible should it be? Should the description also allow to define e.g. a polar coordinate system?

[letsfindaway]

An initial implementation of this is pushed to https://github.com/letsfindaway/OpenBoard/tree/feat-flexible-ruler-description. It implements the painting of background according to ruling descriptions, the modified Background Palette, saving and loading ruling descriptions in SVG document pages and an initial set of 12 different ruling descriptions, 6 of them are shown in the palette.

The branch does currently not contain a preferences page to select the set of background rulings for the palette.

[letsfindaway]

Widget on the preferences page is now available and added to the branch. A Draft PR was created to collect more input on usability.

[letsfindaway]

Draft status on PR now removed. A description is available in the wiki: https://github.com/letsfindaway/OpenBoard/wiki/Flexible-background-patterns