feat(prefs): add word characters preference

[royy92]

What

Add a new Preferences field for configuring the word-chars setting.

Why

Issue #2304 points out that users currently have no way to configure the word-chars value from the Preferences window.

This also makes the setting easier to manage when using --save-preferences and --load-preferences.

How

• Add a new word_chars_entry field to guake/data/prefs.glade.
• Load, update, and reset the word-chars setting in guake/prefs.py.
• Add a release note in releasenotes/notes/word-chars-preference-9fc892802ca01441.yaml.

Testing

• Ran make style.
• Ran make check.
• Ran make test (30 tests passed).
• Verified that changing the field updates the word-chars setting.
• Verified that resetting Compatibility preferences restores the default word-chars value.

Screenshot

The new Word characters preference available in the Compatibility section.

Closes #2304

[Davidy22]

Did a test to make sure it works, noticed that it doesn't immediately apply, only on next startup. May be unintuitive.

I was going to comment on the change to the comment as well but the shortened version grew on me on the second reading. Not sure why you did the label like that though, there are others with underscores but we have them conventionally at the front not middle.

[Davidy22]

Why the underscore in the middle?