fix tasklist indicator line on right panel

[samlane-ma]

Description

When Icon Tasklist is on a right panel, a diagonal line shows through the active item. This PR just tweaks this to make the indicator line vertical.

Closes #592

Submitter Checklist

• [x] Squashed commits with git rebase -i (if needed)
• [x] Built budgie-desktop and verified that the patch worked (if needed)