Icon aliases cleanup

[Martin Měřinský]

Hello,
I'm working towards icons code cleanup and the possibility to create
other, nicer, scalable icons. Possibility to change icons in Subsurface
at will. And finaly usage of your current GUI icons in Subsurface,
where meaningfull.

This first step is using relative icon path and icon aliases from Qt
Resource System. Some of Subsurface icons already use it, some don't.

Up to this point, nothing is changed in GUI.

Please have a look. I will welcome the feedback.
https://github.com/Subsurface-divelog/subsurface/pull/893

Thank you in advance
Martin M.

PS: I reworked this patch according to some previous feedback (used
better names and avoid sorting qrc file).