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/doom/autoload/doom.el

@@ -1,28 +0,0 @@
-;;; ui/doom/autoload/doom.el -*- lexical-binding: t; -*-
-
-;;;###autoload
-(defun +doom/reset-theme ()
-  "Reset the color theme currently in use."
-  (interactive)
-  (let ((theme (or (car-safe custom-enabled-themes) +doom-theme)))
-    (when theme
-      (mapc #'disable-theme custom-enabled-themes))
-    (load "doom-themes-common.el" nil t)
-    (+doom|init)
-    (+doom|refresh-bright-buffers)))
-
-;;;###autoload
-(defun +doom|restore-bright-buffers (&rest _)
-  "Restore `doom-buffer-mode' in buffers when `persp-mode' loads a session."
-  (dolist (buf (persp-buffer-list))
-    (with-current-buffer buf
-      (+doom|buffer-mode-on))))
-
-;;;###autoload
-(defun +doom|refresh-bright-buffers ()
-  "Refresh `doom-buffer-mode', in case of graphical glitches."
-  (dolist (win (window-list))
-    (when (buffer-local-value 'doom-buffer-mode (window-buffer win))
-      (with-selected-window win
-        (doom-buffer-mode -1)
-        (doom-buffer-mode +1)))))