Revert "Added `yii\base\Module::getParam($name, $defaultValue = null)`  to conveniently work with application parameters"

This reverts commit 3b95aa9c.

Added `yii\base\Module::getParam($name, $defaultValue = null)`  to conveniently work with application parameters

Fixes #2303: Fixed the bug that `yii\base\Theme::pathMap` did not support dynamic update with path aliases

see https://github.com/yiisoft/yii2/commit/0cfd41a21a8d89cc577590a253893542e60d43ce#commitcomment-5238567
issue #2233

fixes #2233

Fixes #706: Added `yii\widgets\Pjax` and enhanced `GridView` to work with `Pjax` to support AJAX-update

realpath(false) = current working directory.
This can cause problems with getAlias() which returns false.

see yiisoft/yii#3113

Allow writing widgets using return instead of echo in the run method to make it consistent with controller

- i18n now falls back to `en` from `en-US` if message translation isn't found
- View now falls back to `en` from `en-US` if file not found
- Default `sourceLanguage` and `language` are now `en`

fixes #1984

Fixes #1965: `Controller::findLayoutFile()` returns incorrect file path when layout name starts with a slash