Add rofi theme scaffolding

Themes are structured in two parts: layout and colorschemes. Both can be
set independently from each other. Layouts can make use of anchored
color values (@background, @background-focus,.. look at colorscheme
files for all available values). Colorscheme files then translate the
values into actual colorcodes.

That way, the colorscheme for all layouts can be changed with one
setting, or independently of each other. Layouts try to specify some
often used structures for rofi menus - right now there is a horizontal
list and a fullscreen options selection (which can make use of icon
fonts).

Any global theme changes should be done in settings.rasi. Any global
function changes can still be done in config.rasi. A rofi-powermenu has
been added as an example of using the theming structure. The powermenu
script loads rofi with the powermenu.rasi theme enabled. The powermenu
theme loads the fullscreen options layout, which in turn loads the
vertical list layout, which loads settings, which sets the correct
colors. Idea and original structure from:
https://gitlab.com/vahnrr/rofi-menus

.config/rofi/modes/rofi-powermenu

@@ -0,0 +1,37 @@
+#!/usr/bin/sh
+
+rofi_command="rofi -theme themes/powermenu.rasi"
+
+### Options ###
+power_off=""
+reboot=""
+lock=""
+suspend_btn="鈴"
+logout_btn=""
+# Variable passed to rofi
+
+chosen="$(printf "%s\n%s\n%s\n%s\n%s" "$power_off" "$reboot" "$lock" "$suspend_btn" "$logout_btn" | $rofi_command -dmenu -selected-row 2)"
+case $chosen in
+$power_off)
+	printf "poweroff"
+	# systemctl poweroff
+	;;
+$reboot)
+	printf "reboot"
+	# systemctl reboot
+	;;
+$lock)
+	printf "lock"
+	# light-locker-command -l
+	;;
+$suspend_btn)
+	printf "suspend"
+	# mpc -q pause
+	# amixer set Master mute
+	# systemctl suspend
+	;;
+$logout_btn)
+	printf "logout_btn"
+	# i3-msg exit
+	;;
+esac