Support reading category from in-buffer-settings.

src/context/global_settings.rs

@@ -12,6 +12,8 @@ pub struct GlobalSettings<'g, 's> {
     pub radio_targets: Vec<&'g Vec<Object<'s>>>,
     pub file_access: &'g dyn FileAccessInterface,
     pub in_progress_todo_keywords: BTreeSet<String>,
+    pub category: Option<String>,
+
     pub complete_todo_keywords: BTreeSet<String>,
     /// Set to true to allow for plain lists using single letters as the bullet in the same way that numbers are used.
     ///
@@ -39,6 +41,7 @@ impl<'g, 's> GlobalSettings<'g, 's> {
                 working_directory: None,
             },
             in_progress_todo_keywords: BTreeSet::new(),
+            category: None,
             complete_todo_keywords: BTreeSet::new(),
             list_allow_alphabetical: false,
             tab_width: DEFAULT_TAB_WIDTH,