add functionality to set custom names for tvshow seasons

The first commit adds the functionality to allow users to set a custom name/title for tvshow seasons. This has been requested quite a few times in the forum and it's a rather small change. The seasons will still be sorted by season number independent of the name/title that has been set for a season.

The second commit moves the "Choose art" context button for tvshow seasons from the main context menu to the "Manage..." sub-menu which was introduced by the first commit (to support "Edit title").