Feature request: Ruler

[SimonBauer-git]

First of all, thanks to anyone that contributed to the development of rnote. I have an Idea/Request to add: a ruler with angles that scales with the background (like if you have a grid that every sqaure of the grid is one mark on the ruler no matter the zoom level). That would make this the be all and end all notetaking app for me, and i wouldnt need xournalpp anymore when i need a ruler once a year or so.

[flxzt]

duplicate of #147