[Feature] Importing custom difficulty table and create its folder.

[Unengine]

It will significantly reduce LR2's barrier to entry.

Two tables should be added into song.db

Table Schemas

nexus_playlist - contains data of a difficulty table

• id : identifier of a difficulty table
• name : name of a folder
• symbol : symbol of a difficulty table's level (e.g. ★, ★★, sl, st)
• page_url : source url of a difficulty table

nexus_playlist_entry - contains charts' data in difficulty tables

• id : identifier to know which difficulty table the chart belongs to (related to nexus_playlist's id)
• md5 : identifier of a chart (md5 hash)
• folder : identifier to know which folder the chart belongs to (e.g. ★1, ★2, st0, st1 ...)

Implementation method

1. Get header.json's url from a <difficulty_table.html>'s metadata. (declared as "bmstable")
2. Get the difficulty table's data in the header and process it into a row into nexus_playlist table.
3. Also get course data from the header, and create its lr2folder.
4. Get content.json's url from the header, and get every chart's data inside it, and process it into rows into nexus_playlist_entry table.