[applets/digital-clock] Increase size of calendar in pop-up

Summary:
Various people were complaining that the calendar was too squished after D27903. This
patch restores its size to be closer to the old version. As a consequence, the whole
expanded representation is quite large now, but there really is no other way.

Test Plan:
Before: {F8181551}
After: {F8181550}

Reviewers: #vdg, broulik, mart, cblack

Reviewed By: #vdg, cblack

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28104