mu4e-headers: implement mu4e-headers-toggle-setting

Make it easier to toggle various header-view settings and display them in the mode-line.

NEWS.org

@@ -91,9 +91,6 @@
 
 *** mu4e
 
-   - the old mu4e-view is *gone*; only the gnus-based one remains. This allowed
-     for removing quite a bit of old code.
-
    - the mu4e headers rendering is much faster (a factor of 3+), which makes
      displaying big results snappier. This required some updates in the headers
      handling and in the server protocol. Separate from that, the cached
@@ -103,6 +100,27 @@
    - "related" messages are now recognizable as such in the headers-view, with
      their own face, ~mu4e-related-face~; by default with an italic slant.
 
+   - For performance testing, you can set the variable
+     ~mu4e-headers-report-render-time~ to ~t~ and ~mu4e~ will report the
+     search/rendering speed of each query operation.
+
+   - Removed header-fields ~:attachments~, ~:signature~, ~:encryption~ and
+     ~:user-agent~. They're obsolete with the Gnus-based message viewer.
+
+   - The various "toggles" for the headers-view (full-search, include-related,
+     skip-duplicates, threading) were a bit hard to find and with non-obvious
+     key-bindings. For that, there is now ~mu4e-headers-toggle-setting~ (bound to
+     ~M~) to handle all of that. The toggles are also reflected in the mode-line;
+     so e.g. 'RTU' means we're including [R]elated messages, and show [T]hreads,
+     skip duplicates ([U]nique).
+
+   - the old mu4e-view is *gone*; only the gnus-based one remains. This allowed
+     for removing quite a bit of old code.
+
+   - A new ~defcustom~, ~mu4e-view-open-program~ for starting the appropriate
+     program for a give file (e.g., ~xdg-open~). There are some reasonable
+     defaults for various systems. This can also be set to a function.
+
    - indexing happens in the background now and mu4e can interact with the
      server while it is ongoing; this allows for using mu4e during lengthy
      indexing operations.
@@ -122,14 +140,6 @@
      automatically change the ~Organization:~ header, and attempts to update the
      message signature. Also, there's a key-binding now: ~C-c ;~
 
-   - A new ~defcustom~, ~mu4e-view-open-program~ for starting the appropriate
-     program for a give file (e.g., ~xdg-open~). There are some reasonable
-     defaults for various systems. This can also be set to a function.
-
-   - For performance testing, you can set the variable
-     ~mu4e-headers-report-render-time~ to ~t~ and ~mu4e~ will report the
-     search/rendering speed of each query operation.
-
    - Changed the default for ~mu4e-compose-complete-only-after~ to 2018-01-01, to
      filter out contacts not seen after that date.
 
@@ -143,9 +153,6 @@
 
    - Removed ~make-mu4e-bookmark~ which was obsoleted in version 1.3.9.
 
-   - Removed header-fields ~:attachments~, ~:signature~, ~:encryption~ and
-     ~:user-agent~. They're obsolete with the Gnus-based message viewer.
-
    - Add command ~mu4e-sexp-at-point~ for showing/hiding the s-expression for the
      message-at-point. Useful for development / debugging. Bound to ~,~ in headers
      and view mode.