Refactor the page-message handling

[helkv]

Page-messages display the success or the failure of many actions. Because they cover other elements for some time, they must be handled correctly in the test-methods.

Actual Approach:

• to avoid ElementClickIntercepted-failures, the messages are hidden, when they cover other elements that will be clicked
• the hideMessages() method hides the message, by setting the display style to none (Handle test failures caused by message area #31)

Problems:

• sometimes it is not obvious why the hideMessages() method is called (e.g. at the end of a test, when the message would cover elements to be clicked in the following test)
• the hideMessages() method is called very often inside test-methods, which unnecessary enlarges the test-methods

TODO:

• use a more generic approach