[Web Browser] Use more appropriate panel icon and increase default size

Summary:
If you add an applet not to the desktop, but to the panel, then the old icon is out of the general design of the panel, and the size of the browser window does not allow reading the contents. Changes fix this.

Reviewers: ematirov, Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19417