Set explicit minimum size on panelSpacer so that AppletContainer doesn't set one
Summary:
Commit 8e1339887a01e078b04fb18ff575c16998447665 in Plasma 5.8 removed a
minimum size on the panel spacer, which is what we want, however if no
minimum size is set on an applet in a panel the panel's AppletContainer
sets the minmimum width to be the same as the height.
This was preventing people having tiny panel spacers.
BUG: 369823
Test Plan: Reporter tested the patch
Reviewers: Plasma
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D2938