[Default Tooltip] Fix sizing

Unset the Label's default height which completely confuses the renderer either shifting the item vertically (e.g. when toggling
system tray popup), or letting it get too wide (e.g. when moving from a two-line tooltip to a one-line tooltip).
Also let the Layouts figure out sizing rather than trying to overrule it with flaky values.

BUG: 386260
BUG: 389371

Differential Revision: https://phabricator.kde.org/D10077