applets/notification: fix history view vertical lines
Right now we use a thickened version of the Plasma theme's vertical line SVG for the lines in grouped history items. However this is an abuse of the SVGs, which were not necessarily designed to be thickened in that way. The result looks fine for some themes where the SVG really is just a plain vertical line, but does not look good for themes where the graphic is fancier--for example, Oxygen.
Since the look we want here is just a thick line (AKA a rectangle), let's just create one ourselves to avoid drama with the Plasma theme.
BUG: 416386 FIXED-IN: 5.27.5
No befores-and-afters since this is mostly a correctness fix that results in no visual changes when using the Breeze Plasma theme. The visual change when using the Oxygen theme is just that the vertical rectangles are now actual vertical rectangles and no longer weird glitchy blobs.