sourceview: improve input-method detection for indenters

This attempts to improve the situation where we need to detect if an input method filtered our keypress but really inserted what we expected it to. This can happen with ibus over wayland stealing input like > or / and marking it filtered (but going through "commit-text").

To make this work, we have to introspect a bit more by trying to convert our keyval to a unicode character, and then checking equality with what was just inserted at the previous character.

Fixes #282 (closed)