The graphview widget now implements the gtk::Scrollable interface, so it is no longer wrapped inside a gtk::Viewport when used in a gtk::ScrollWindow anymore.
Instead, it repositions its content itself when scrolled, and also skips rendering any content that is not inside the visible area, which should improve performance
when the graph becomes big.
This commit also makes the canvas a fixed size, with much space to each side from the starting area.
This will hopefully improve user experience, as the view can now be moved around more freely, and nodes can be dragged left and above the starting area.
This makes gtk draw the background grid for us via CSS, instead of manually drawing each line via cairo.
This improves performance, as the grid may now be drawn via GPU, and gets rid of the custom drawing code we had.
This removes the manual painting of the background via cairo and adds the correct color to CSS instead,
which should hopefully improve performance as we do less cpu painting like this.
For good contrast in both dark and light mode, the link color is now a semi-light gray instead of complete black, which had bad contrast in dark mode and good constrast in light mode.
Later, we can seperate color palettes for light and dark mode, but only together with a dark mode toggle button or system-wide darkmode toggle.
Previously, these were defined directly in the code,
but defining them in the css helps seperating theming and behaviour
and makes the colors easier to tweak.
