[KCM]Set implicitWidth for main page
Summary:
Set implicitWidth in the main page for the initial size when opening
audio settings.
Test Plan:
Open audio settings from kickoff.
Before:
After:
Reviewers: Plasma, VDG, drosca, ngraham
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D27648