Improve KFloppy icon

Summary:
The KFloppy icon has really weird proportions, making the floppy appear fat.

This patch fixes that and makes other minor changes to improve the icon's aesthetics

"Why?" you might ask? Because this icon is going to be the basis of a colorful "Save"
icon for https://bugs.kde.org/show_bug.cgi?id=412404.

CCBUG: 412404

Test Plan:

Reviewers: VDG, ndavis, wbauer

Reviewed By: VDG, ndavis, wbauer

Subscribers: wbauer, ndavis, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24314