Clean up and optimize popups-{ltr,rtl}.svg

Using Inkscape, svgo and by hand. Removed random leftover filters,
cleaned up the grouping structure.

Replaced popups-ltr.svg with a flipped version of popups-rtl.svg,
as the former caused some inexplicable issues with some renderers
(like rsvg).

Change-Id: Id67c26cd8ac71a8fc5de3f2fc1cc803482e86699