Use QStandardPaths instead of hard coded paths

[dcaliste]

Close #20

Also adjust some icon sizes, since large version are not existing for up and down.

[Olf0]

@dcaliste, wow, thanks a lot!

Side not / question: Have you addressed all hardcoded /home/nemo paths, or shall I still check if there are some remaining to be addressed?

[dcaliste]

Normally, I've addressed all cases. Not all old paths are now treated 1:1 with the QStandardPaths approach. I'm thinking for instance to the database location, which was previously in ~/.config/, but I've now moved it to ~/.config/flowplayer/flowplayer (the name redundancy is due to the way APP_CONFIG path is created with a style like org/app, and here the organisation is declared as flowplayer as for the application name). This latter path is compatible with Sailjail for instance.