Make --doctool locale aware

* Adds `indent(str)` to `String`:
    * Indent the (multiline) string with the given indentation.
    * This method is added in order to keep the translated XML correctly
      indented.
* Moves the loading of tool/doc translation into
  `editor/editor_translation.{h,cpp}`.
    * This will be used from both `EditorSettings` and the doc tool from
      `main`.
* Makes use of doc translation when generating XML class references, and
  setup the translation locale based on `-l LOCALE` CLI parameter.

The XML class reference won't be translated if `-l LOCALE` parameter is
not given, or when it's `-l en`.

doc/classes/String.xml

@@ -124,7 +124,7 @@
 		<method name="dedent" qualifiers="const">
 			<return type="String" />
 			<description>
-				Returns a copy of the string with indentation (leading tabs and spaces) removed.
+				Returns a copy of the string with indentation (leading tabs and spaces) removed. See also [method indent] to add indentation.
 			</description>
 		</method>
 		<method name="ends_with" qualifiers="const">
@@ -243,6 +243,15 @@
 			<description>
 			</description>
 		</method>
+		<method name="indent" qualifiers="const">
+			<return type="String" />
+			<argument index="0" name="prefix" type="String" />
+			<description>
+				Returns a copy of the string with lines indented with [code]prefix[/code].
+				For example, the string can be indented with two tabs using [code]"\t\t"[/code], or four spaces using [code]"    "[/code]. The prefix can be any string so it can also be used to comment out strings with e.g. [code]"# "[/code]. See also [method dedent] to remove indentation.
+				[b]Note:[/b] Empty lines are kept empty.
+			</description>
+		</method>
 		<method name="insert" qualifiers="const">
 			<return type="String" />
 			<argument index="0" name="position" type="int" />