Fix multiline descriptions

Created by: ederag

Solves #413 (closed), that was actually hidden before #412. By the way, fixed row height to avoid tag spill-out such as

Also fixed the category color. Now it has always a lower contrast, even when the row is selected. And that works for other themes (tested with dark adwaita). Under adwaita the new color is within 5% of the old one.

Developer-wise, the margins are defined in variables, instead of hard-coded. Changing one of these variables value should still give consistent results.