[Emacs] Change font to DejaVu Sans Mono, better mu4e visual config

This commit changes the font of Emacs from Fantasque Sans Mono to
DejaVu Sans Mono. The latter has a better support for the
International Phonetic Alphabet (IPA) than the former. Font size is
also adjusted.

In the mu4e header view, the width of the header view when a message
is opened is now better handled since it will be calculated
accordingly to the frame and not the window. In Emacs, the frame is
actually what we commonly call a window, and a window is the area in
which buffers are displayed (damn this pre-GUI terminology in Emacs).
There is a bug with Emacs launched as a server though: since it is
launched as a headless service at first, the edited code cannot know
what the size of the frame will be, so the user will have to
re-evaluate this code snippet once the first frame is created, or when
the frame is resized and the user wishes to see this value updated.

A possible fix for this could be to hook this evaluation to when a
message is opened instead of when Emacs spins up.

org/config/spacemacs.org

@@ -854,8 +854,8 @@
     The below  variable sets either  the default font  or a prioritized  list of
     fonts to be used by Emacs.
     #+BEGIN_SRC emacs-lisp
-      (setq-default dotspacemacs-default-font '("Fantasque Sans Mono"
-                                                :size 10.0))
+      (setq-default dotspacemacs-default-font '("DejaVu Sans Mono for Powerline"
+                                                :size 9.0))
     #+END_SRC
 
     I also added the following code in order to define a fallback font for
@@ -3229,7 +3229,7 @@
    40% of Emacs’ frame, the rest will be given to emails.
    #+BEGIN_SRC emacs-lisp
      (setq mu4e-split-view 'vertical
-           mu4e-headers-visible-columns (round (* (window-total-width)
+           mu4e-headers-visible-columns (round (* (frame-total-cols)
                                                   0.4)))
    #+END_SRC