Dear Maintainer,

When setting the indentation to use "Tab" in both Preferences and Document
Properties, the Document Properties setting reverts to "Space" after
closing
and reopening the application. This causes the editor to use spaces instead
of tabs, which overrides the Preferences setting.

Steps to reproduce:
1. Open gnome-text-editor
2. Go to Preferences → Indentation → set "Character" to "Tab"
3. Go to Document Properties → set "Character" to "Tab"
4. Close the application completely
5. Reopen gnome-text-editor
6. Check Document Properties → "Character" is now set to "Space" again
7. Check Preferences → "Character" is still set to "Tab" correctly

Expected behavior: Both settings should persist as "Tab" after restart, or
Document Properties should not override Preferences.

Actual behavior: Document Properties reverts to "Space" after restart and
this setting overrides the Preferences setting, forcing spaces even though
Preferences says "Tab".

This is particularly problematic for Python development where mixing tabs
and spaces causes "TabError: inconsistent use of tabs and spaces in
indentation".