Refine debug logging

[kanru]

Is your feature request related to a problem? Please describe.

The current logging for key events is a little too verbose and not easy to read.
We should redo the logging after we refactor the key event handling module.

Current

[DEBUG chewing::editor] process_keyevent: key-K49-Esc-�-
[DEBUG chewing::editor] process_keyevent: key-K32-H-h-
[DEBUG chewing::editor] process_keyevent: key-K34-K-k-
[DEBUG chewing::editor] process_keyevent: key-K4-N4-4-
[DEBUG chewing::editor] process_keyevent: key-K31-G-g-
[DEBUG chewing::editor] process_keyevent: key-K4-N4-4-
[DEBUG chewing::editor] process_keyevent: key-K52-Backspace-�-
[DEBUG chewing::editor] process_keyevent: key-K52-Backspace-�-
[DEBUG chewing::editor] process_keyevent: key-K49-Esc-�-
[DEBUG chewing::editor] process_keyevent: key-K49-Esc-�-
[DEBUG chewing::editor] process_keyevent: key-K49-Esc-�-
[INFO  chewing::editor] Set syllable editor: Standard { syllable: Syllable { value: 32768, to_string(): "" } }
[DEBUG chewing::editor] process_keyevent: key-K49-Esc-�-
[DEBUG chewing::editor] process_keyevent: key-K49-Esc-�-