[KCM] Display scale factor as a percentage, not a decimal

Summary:
This is the way GNOME and Windows present this information, and I think it's clearer
than using a decimal value.

The underlying value is still a float, we just convert to an int in percentage form for
display purposes.

Test Plan: {F7500851}

Reviewers: #vdg, #plasma, romangg, ndavis

Reviewed By: #vdg, #plasma, romangg, ndavis

Subscribers: ndavis, dhaumann, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24370