Change how themes, fonts & modelines are loaded

WARNING: THIS IS A BREAKING CHANGE FOR THEME/FONT/NLINUM CUSTOMIZATIONS.

This change was motivated by the need to decouple theme and font loading
from the ui/doom module.

Now, it is doom-core's purview. Theme and fonts are loaded after
initfiles are read (attached to the doom-init-ui-hook hook), giving
other modules (especially private ones) a chance to change the theme or
fonts.

+ Refactor core-ui.el
+ New init hook: doom-init-ui-hook
+ Decouple theme/font loading from ui/doom
+ Load modelines are doom-init-ui-hook
+ New theme/font variables (replaces old ui/doom variables)
  + doom-theme
  + doom-font
  + doom-variable-pitch-font
  + doom-unicode-font
+ Change nlinum variables
  + doom-line-number-lpad
  + doom-line-number-rpad
  + doom-line-number-pad-char

Addresses #117

core/core-popups.el

@@ -164,7 +164,7 @@ for :align t on every rule."
           (t
            (when doom-popup-no-fringes
              (set-window-fringes window
-                                 doom-ui-fringe-size doom-ui-fringe-size
+                                 doom-fringe-size doom-fringe-size
                                  fringes-outside-margins))
            ;; Ensure window parameters are cleaned up
            (set-window-parameter window 'popup nil)