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/modules/settings_common.lua

@@ -0,0 +1,84 @@
+-- Common helpers for settings pages (res/layouts/pages/settings_*.xml.lua).
+--
+-- Usage in a page script:
+--
+--   local settings = require "core:settings_common".new(document, {
+--       tostring_overrides = {["display.framerate"] = function(x) ... end}
+--   })
+--   -- 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
+
+local settings_common = {}
+
+--- Create helpers bound to the page document.
+--- @param document the page document
+--- @param options table (optional) with fields:
+---   tostring_overrides: {[setting_id] = function(value) -> string}
+function settings_common.new(document, options)
+    options = options or {}
+    local tostring_overrides = options.tostring_overrides or {}
+    local this = {}
+
+    --- Update trackbar label (called by the track_setting template)
+    function this.update_trackbar_label(x, id, name, postfix)
+        local str
+        local func = tostring_overrides[id]
+        if func then
+            str = func(x)
+        else
+            str = app.str_setting(id)
+        end
+        document[id..".L"].text = string.format(
+            "%s: %s%s",
+            gui.str(name, "settings"),
+            str,
+            postfix
+        )
+    end
+
+    --- Add a trackbar bound to the setting
+    --- @param changeonrelease apply value only when mouse is released
+    function this.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
+        }))
+        this.update_trackbar_label(app.get_setting(id), id, name, postfix)
+    end
+
+    --- Add a checkbox bound to the setting
+    --- @param consumer name of a global page function(id, value) called on
+    --- change instead of just setting the value (optional)
+    function this.create_checkbox(id, name, tooltip, consumer)
+        tooltip = tooltip or ""
+        local action
+        if consumer then
+            action = string.format("%s(\"%s\", x)", consumer, id)
+        else
+            action = string.format("app.set_setting(\"%s\", x)", id)
+        end
+        document.root:add(string.format(
+            "<checkbox consumer='function(x) %s end' checked='%s' tooltip='%s'>%s</checkbox>",
+            action, app.str_setting(id), gui.str(tooltip, "settings"), gui.str(name, "settings")
+        ))
+    end
+
+    return this
+end
+
+return settings_common