Simplification: clarify Upload Score functionality

[keckelt]

Part of #192

The "Upload Score" function needs to be described better; from the button & description, it's not clear what it does:

• is something uploaded from ordino to the cloud?
• or do you upload something to ordino?

--> maybe rename to import score

In the dialogue, specify: what files can be uploaded, how they have to be formatted (delimiters) and what data has to be inside of it:

• only one score, or can it contain multiple?
• is the score inserted in the order of the file, or do you need ids
• if ids, does it have to be the first column or are column headings needed?

Is the uploaded score saved or temporary? if saved, where?