Move commands in webdriver.py into other mixin's classes #325

We'd like to reduce a number commands in webdriver.py, since the file has a bunch of commands in one file

example: #307

https://github.com/appium/python-client/blob/master/appium/webdriver/webdriver.py#L741

• LAUNCH_APP, CLOSE_APP
  • => https://github.com/appium/python-client/blob/master/appium/webdriver/extensions/applications.py
• GET_CURRENT_PACKAGE, END_TEST_COVERAGE, OPEN_NOTIFICATIONS
  • https://github.com/appium/python-client/blob/master/appium/webdriver/extensions/android/common.py (new file)
• SET_IMMEDIATE_VALUE, REPLACE_KEYS, CLEAR
  • https://github.com/appium/python-client/blob/master/appium/webdriver/extensions/keyboard.py
  • Maybe keyboard is enough since the three also type text