UI: Expand the "Units" configuration menu to include "Pixels"

Rename it to "Dimension units", and make it a radio submenu.

Migrate the previous metric_length setting to this new dimension_units
one.

Update optionsutil to follow it.

frontend/ui/data/onetime_migration.lua

@@ -10,7 +10,7 @@ local util = require("util")
 local _ = require("gettext")
 
 -- Date at which the last migration snippet was added
-local CURRENT_MIGRATION_DATE = 20240914
+local CURRENT_MIGRATION_DATE = 20240915
 
 -- Retrieve the date of the previous migration, if any
 local last_migration_date = G_reader_settings:readSetting("last_migration_date", 0)
@@ -722,5 +722,15 @@ if last_migration_date < 20240914 then
     G_reader_settings:delSetting("save_document")
 end
 
+-- 20240915, metric_length -> dimension_units, https://github.com/koreader/koreader/pull/12507
+if last_migration_date < 20240915 then
+    logger.info("Performing one-time migration for 20240915")
+
+    if G_reader_settings:has("metric_length") then
+        G_reader_settings:saveSetting("dimension_units", G_reader_settings:nilOrTrue("metric_length") and "mm" or "in")
+        G_reader_settings:delSetting("metric_length")
+    end
+end
+
 -- We're done, store the current migration date
 G_reader_settings:saveSetting("last_migration_date", CURRENT_MIGRATION_DATE)

frontend/ui/data/optionsutil.lua

@@ -17,10 +17,7 @@ function optionsutil.enableIfEquals(configurable, option, value)
     return configurable[option] == value
 end
 
--- Converts px size to mm, inch or pt
--- if the `metric_length`-setting is not set or true -> mm
--- if the `metric_length`-setting is false -> inch
--- if format == "pt" -> pt
+-- Converts flex px/pt sizes to absolute px, mm, inch or pt
 local function convertSizeTo(px, format)
     local format_factor
 
@@ -75,7 +72,7 @@ function optionsutil.showValues(configurable, option, prefix, document, unit)
     local value_default, value_current
     unit = unit or option.name_text_unit
     if unit and unit ~= "pt" then
-        unit = G_reader_settings:nilOrTrue("metric_length") and "mm" or "in"
+        unit = G_reader_settings:readSetting("dimension_units", "mm")
     end
     if option.toggle and option.values then
         -- build a table so we can see if current/default settings map
@@ -180,7 +177,7 @@ end
 function optionsutil.showValuesHMargins(configurable, option)
     local default = G_reader_settings:readSetting("copt_"..option.name)
     local current = configurable[option.name]
-    local unit = G_reader_settings:nilOrTrue("metric_length") and "mm" or "in"
+    local unit = G_reader_settings:readSetting("dimension_units", "mm")
     if not default then
         UIManager:show(InfoMessage:new{
             text = T(_([[

frontend/ui/elements/common_settings_menu_table.lua

@@ -716,20 +716,15 @@ common_settings.keyboard_layout = {
 common_settings.font_ui_fallbacks = dofile("frontend/ui/elements/font_ui_fallbacks.lua")
 
 common_settings.units = {
-    text = _("Units"),
+    text_func = function()
+        local unit = G_reader_settings:readSetting("dimension_units", "mm")
+        return T(_("Dimension units: %1"), unit)
+    end,
     sub_item_table = {
-        {
-            text = _("Metric length"),
-            checked_func = function()
-                return G_reader_settings:readSetting("metric_length", true)
-            end,
-            callback = function(touchmenu_instance)
-                G_reader_settings:toggle("metric_length")
-                if touchmenu_instance then touchmenu_instance:updateItems() end
-            end,
-            keep_menu_open = true,
-        },
-    },
+        genGenericMenuEntry(_("Metric system"),   "dimension_units", "mm", nil, true),
+        genGenericMenuEntry(_("Imperial system"), "dimension_units", "in", nil, true),
+        genGenericMenuEntry(_("Pixels"),          "dimension_units", "px", nil, true),
+    }
 }
 
 common_settings.screenshot = {