[SUG][PLAYER] Cleanse a single level

[namida]

Currently, cleansing a single level requires a somewhat tedious workaround: Create a new pack (it can just be a folder in "levels" though, at least, with no other metadata), put the single level in it, and run Cleanse on it as a pack.

A button should be added to run Cleanse on a single level at once.

Related to this, it should also be possible to cleanse one (or more?) levels via command line, for future editor usage or just for people who like to script stuff.

[namida]

Implemented in commit e87c972. Will be included in 12.10.1.