if the user did set an implicit size, keep it

Summary:
if the user code did something like
IconItem {

implicitWidth: 32
implicitHeight: 32

}

then never automatically update implicitWidth or height.
this favors compatibility and fixes the desktop
toolbox appearance

Test Plan:
textbox works, other icon items in plasmashell seems properly sized.
applet alternatives fix is still needed

Reviewers: Plasma, hein

Reviewed By: Plasma, hein

Subscribers: hein, davidedmundson, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D5243