Add fileSelectionHandlerOptions option to FileInputView

Allow supplying further options to be passed in the payload to allow
the file selection handler to determine its referer dynamically.
Add returnToTab option to PageConfigurationFileSelectionHandler

By default one returns to the files tab after selecting a file to be
used in a page. This option allows to specify a different tab to
return to.
Make TabsView ignore unknown defaultTab

Do not fail if the `defaultTab` references an unknwon tab name. Use
first tab as fallback.
Remove window onload timeout

pull request #119 introduced a timeout which resolves the ready
promise if window.onload is not fired after 10 seconds, to solve #118
in Safari.  This change introduced a new problem though: When the dom
ready event takes longer than 10 seconds we start while some of the
widgets are not yet defined. This happens mostely in
development. Still, since #131 solved the onload issue in Safari, we
revert the timeout hack.
Merge pull request #166 from tf/improve-tabs-view

Improve TabsView
Merge pull request #167 from tf/revert-onload-timeout

Remove window onload timeout