[Python] Add methods clear for actions

[bayandin]

Added method clear for ActionChains and TouchActions to clear stored actions. It does not break backward compatibility and design.
As a continuation of issues #99.

CLA signed as Alexander Bayandin

[shs96c]

Why not just create a new instance of the ActionChains? After all, a cleared instance is an empty instance.

[bayandin]

In my opinion, it is better to reuse than to create each time a new instance of the object.

For example:

ActionChains(driver).some_chain_of_actions(elements).perform()
ActionChains(driver).some_other_chain_of_actions(elements).perform()
ActionChains(driver).and_some_other_chain_of_actions(elements).perform()

VS

chain = ActionChains(driver)

chain.some_chain_of_actions(elements).perform().clear()
chain.some_other_chain_of_actions(elements).perform().clear()
chain.and_some_other_chain_of_actions(elements).perform()

And store actions after performing is a unpredictable result for some situations (see #99).

[barancev]

Wha't the principal difference between a "cleaned" instance and a new one?

[bayandin]

There is no difference except the small overhead of creating an object.

[barancev]

This is an excessive optimization that does not worth adding a new method to the API.

[richardmodis]

It's a pity that this implementation wasn't accepted. For instance, I use ActionChains a lot of time, and I could create much more readable (and shorter) code by reusing the instance.

[andreastt]

We'd like to actively encourage people to build optimizations like this on top of the WebDriver libraries, though. As you say it could allow for more readable and less verbose code, but in the eyes of WebDriver it bloats the API for very little benefit.