[applets/digital clock] Make tooltip spacing and text nicer and more consistent

Test Plan:
Before, no clocks: {F8246116}
After, no clocks: {F8246114}
[Now the vertical spacing between labels matches other tooltips]

Before, some clocks: {F8246113}
After, some clocks: {F8246115}
[Now the full date is displayed on the first line instead of being split across two]

Reviewers: #vdg, #plasma, ndavis

Reviewed By: #vdg, ndavis

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28970