support configurable middle click scoll button

I'm using button 12, which on my Elecom HUGE trackball is labeled
"Fn3". Existing users can set this variable to 2 if they wish to retain
the previous behaviour.

group_vars/all

@@ -207,7 +207,7 @@ himawaripy:
     run_on: trusted
     flags: --auto-offset --level 4
 
-middle_click_scroll: True
+middle_click_scroll: 12
 
 display_manager: lightdm
 gdm:

roles/x/tasks/main.yml

@@ -30,8 +30,8 @@
     - user
 
 - name: Enable middle-click scrolling
-  copy: src=30-scroll.conf dest=/etc/X11/xorg.conf.d/30-scroll.conf
-  when: middle_click_scroll is defined and middle_click_scroll == True
+  template: src=30-scroll.conf dest=/etc/X11/xorg.conf.d/30-scroll.conf
+  when: middle_click_scroll is defined
 
 - include: sxhkd.yml
 - include: unclutter.yml

roles/x/templates/30-scroll.conf

@@ -2,5 +2,5 @@ Section "InputClass"
         Identifier "libinput middle click scroll"
         Driver "libinput"
         Option "ScrollMethod" "button"
-        Option "ScrollButton" "2"
+        Option "ScrollButton" "{{ middle_click_scroll }}"
 EndSection