[Active Window Control] Slightly fix vertical alignment of window name

Summary:
On my setup the window label was 1px lower than global menu items (with Fill height enabled) which was very annoying.
I guess it was caused by some rounding error. This patch fixes it.

Old:

New:

(Red rectangle was added so the issue is easier to spot.)

Test Plan: Checked that window label is still vertically centered.

Reviewers: Plasma, martinkostolny

Reviewed By: martinkostolny

Subscribers: abetts, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11065