Add custom_modules build option to compile external user modules

This patch adds ability to include external, user-defined C++ modules
to be compiled as part of Godot via `custom_modules` build option
which can be passed to `scons`.

```
scons platform=x11 tools=yes custom_modules="../project/modules"
```

Features:

- detects all available modules under `custom_modules` directory the
same way as it does for built-in modules (not recursive);
- works with both relative and absolute paths on the filesystem;
- multiple search paths can be specified as a comma-separated list.

Module custom documentation and editor icons collection and generation
process is adapted to work with absolute paths needed by such modules.

Also fixed doctool bug mixing absolute and relative paths respectively.

Implementation details:

- `env.module_list` is a dictionary now, which holds both module name as
  key and either a relative or absolute path to a module as a value.
- `methods.detect_modules` is run twice: once for built-in modules, and
  second for external modules, all combined later.
- `methods.detect_modules` was not doing what it says on the tin. It is
  split into `detect_modules` which collects a list of available modules
  and `write_modules` which generates `register_types` sources for each.
- whether a module is built-in or external is distinguished by relative
  or absolute paths respectively. `custom_modules` scons converter
  ensures that the path is absolute even if relative path is supplied,
  including expanding user paths and symbolic links.
- treats the parent directory as if it was Godot's base directory, so
  that there's no need to change include paths in cases where custom
  modules are included as dependencies in other modules.

main/main.cpp

@@ -1645,7 +1645,11 @@ bool Main::start() {
 		print_line("Loading docs...");
 
 		for (int i = 0; i < _doc_data_class_path_count; i++) {
-			String path = doc_tool.plus_file(_doc_data_class_paths[i].path);
+			// Custom modules are always located by absolute path.
+			String path = _doc_data_class_paths[i].path;
+			if (path.is_rel_path()) {
+				path = doc_tool.plus_file(path);
+			}
 			String name = _doc_data_class_paths[i].name;
 			doc_data_classes[name] = path;
 			if (!checked_paths.has(path)) {