TrimHandler: make windows a property of the file.

The main advantage is it can now be used outside of class methods,
though it is not currently used that way.
TrimHandler: fix example comment formatting.
TrimHandler: use log.windowError instead of assert.
TrimHandler: add makePrefix option.

Fixes #13.

If true, the encoding script attempts to create the prefix directory
before the encoding command is run.
Aegisub-Motion: re-align trim interface tables.