[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