[CLEANUP] Invalid return statement in ConfigurationManager

The inconsistent return statement in the loadTabsConfiguration()
method in the ConfigurationManager is removed.
[FEATURE] Fallbacks for determining the current PID

The getCurrentPageUid() method in the Configuration manager is
enhanced and will now use either the first site root or the
first template root page that is found as fallback if other
methods fail to retrieve the current page ID.

This is a workaround for the invalid link handling in the tab
switch of the element browser in TYPO3.

Thanks to tritumRz for the initial pull request!

Resolves: #3