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

modules/ui/unicode/config.el

@@ -1,9 +1,10 @@
 ;;; ui/unicode/config.el -*- lexical-binding: t; -*-
 
-(setq-default bidi-display-reordering t)
-
 (def-package! unicode-fonts
   :demand t
+  :init
+  (setq-default bidi-display-reordering t
+                doom-unicode-font nil)
   :config
   ;; NOTE will impact startup time on first run
   (unicode-fonts-setup))