Share settings page helpers, add GUI cursor/scale API

- move create_trackbar_setting/update_trackbar_label/create_checkbox from
  settings_audio/display/graphics pages to core:settings_common module
- GUI scale trackbar shows the really applied scale, e.g. '3 (effective 2)'
- add gui.get_scale(), gui.get_max_scale(), gui.get_cursor_pos() (UI units);
  document that input.get_mouse_pos() returns window pixels
- InventoryView default color is defined once (InventoryView::DEFAULT_COLOR)
  instead of duplicated literals; 'color' attribute still overrides it

res/layouts/pages/settings_graphics.xml.lua

@@ -1,39 +1,8 @@
-function create_trackbar_setting(id, name, step, postfix, tooltip, changeonrelease)
-    local info = app.get_setting_info(id)
-    postfix = postfix or ""
-    tooltip = tooltip or ""
-    changeonrelease = changeonrelease or ""
-    document.root:add(gui.template("track_setting", {
-        id=id,
-        name=gui.str(name, "settings"),
-        value=app.get_setting(id),
-        min=info.min,
-        max=info.max,
-        step=step,
-        postfix=postfix,
-        tooltip=tooltip,
-        changeonrelease=changeonrelease
-    }))
-    update_trackbar_label(app.get_setting(id), id, name, postfix)
-end
-
-function update_trackbar_label(x, id, name, postfix)
-    -- updating label
-    document[id..".L"].text = string.format(
-        "%s: %s%s",
-        gui.str(name, "settings"),
-        app.str_setting(id),
-        postfix
-    )
-end
-
-function create_checkbox(id, name, tooltip)
-    tooltip = tooltip or ''
-    document.root:add(string.format(
-        "<checkbox consumer='function(x) app.set_setting(\"%s\", x) end' checked='%s' tooltip='%s'>%s</checkbox>",
-        id, app.str_setting(id), gui.str(tooltip, "settings"), gui.str(name, "settings")
-    ))
-end
+local settings = require "core:settings_common".new(document, {})
+-- templates call these functions by name, so they must be global
+create_trackbar_setting = settings.create_trackbar_setting
+update_trackbar_label = settings.update_trackbar_label
+create_checkbox = settings.create_checkbox
 
 function on_open()
     create_trackbar_setting("chunks.load-distance", "Load Distance", 1)