Adjust property names

[flekschas]

Commit or branch: master

The following properties should be renamed:

1. itemPadding should be called cellPadding as the padding isn't applied to items but to the grid cell
2. pileCellAlign should be called pileCellAlignment as we use nouns
3. itemAlignment should be called pileItemAlignment as the alignment
4. itemRotated should be called pileItemRotation because single items are not rotated and because we use nouns