[Weather] Make update time spinbox editable

Test Plan:
Now you can enter text in it using the keyboard. To make this more predictible, the
custom stepSize value is removed so all integer valued between the min and max are
acceptable now.

Reviewers: #vdg, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24815