Add hooks so other apps can display their types

Fix #82

Add docs how to add types, filters and activities

Add a hook to change the icon for activities

Allow other apps to group their activities

Allow other apps to use the ParameterHelper for their activities

Allow apps to use the translation stuff for their activities aswell

Only try to display a file icon when we have a file

Adding \OCP\Activity\IExtentsion and it's first method getNotificationTypes()

Adding method filterNotificationTypes()

Adding method getDefaultTypes()

Adding method translate() and getTypeIcon()

Adding method getGroupParameter()

Adding method getNavigation()

Adding method isFilterValid() and getQueryForFilter()

simplify helper functions getPageFromParam() and getFilterFromParam()

Add unit tests for getFilterFromParam()

Fix Data::getFilterFromParam() test after hardcoding the parameter name

update readme

Format classname as code
switch core branch to stable7 as well