Use consistent ordering for System Tray items

Summary:
@mart dislikes idea of manually ordering the systray (D11233, D11292) but prefers auto
ordering instead. To make my opinion I hacked up this and started testing on myself.
Current version breaks if two items have the same category and text (I guess this
should not happen but...).

Note that category order is more or less randomly chosen (I do believe someone else
has an opinion what the correct order is).

This is mutually exclusive with D11292.

Reviewers: #vdg, #plasma, mart, ngraham

Reviewed By: #vdg, #plasma, mart, ngraham

Subscribers: kmaterka, ognarb, ngraham, wsdfhjxc, mart, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11352