🔥 Fix some compiler warnings

🔥 Fix some compiler warnings

The following warnings have been taken care of:

- Unused “event” parameters, used by Guava’s EventBus to identify event
  listener methods; reworked as extension methods
- A type mismatch caused by an unbounded type parameter
- Deprecated usages of String.capitalize() and .toLowerCase()
- Replace usage of Guava’s deprecated Files.write()
- Unchecked cast (solved by removing the whole piece of code, which was
  unused)
- Some unused parameters and variables
- Uses of ExpectedException in tests; replaced by assertThrows()
- Warning about usage of deprecated getPluginTalker() method was
  suppressed
- Suppressed some more warnings about unchecked casts
- Change a number of casts to wildcard types
- Specified some type parameters explicitely

The warning on the RenderFilter has been intentionally left in, because
at some point I need to figure out _why_ I thought that this expression
could be null.