FIX Repalce mouseDown with keyDown event on treedropdown option button

[sabina-talipova]

Description

Replaced unnecessary mouseDown event with keyDown event to prevent double invoking handleNavigate method. New behat test cases were added to the silverstripe\cms module.

Manual testing steps

Reproduction steps Will require that you have a parent page with at least two levels of child pages

• Create Page with few nested child and grandchild
• Go to any TinyMCE content block on the site
• Select the Insert link and then select Page on this site
• Select the Select a page dropdown
• Select the arrow to view a pages child pages on option for new created Page, if it has more than one child, then you should see child Page and not grandchild Page.

Issues

• https://github.com/silverstripe/silverstripe-cms/issues/2930

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