Ft better export of locale

Working with the locale-page could be better. Here an attempt to make it so:
* Allow to filter all labels
    * not OR backend OR frontend
    * show 'application' in the datagrid when no application is selected in the filter
* Add the 'edited_on' field to the datagrid + make is sortable
    * so a quick overview of the latest changed labels is possible
    * when showing more then one language, the edited_on field shows the most recent 'edited_on' value of the selected languages
* add multi-action-checkbox next to each label (also in multi-language view)
    * This isn't used for anything except: When you click the export-button when certain labels are checked, only these labels are exported.
    * This, in combination with the sortable edited_on field in the datagrid, allows for super fast exporting of the complete locale.xml when creating an installer for a new module
* when clicking on a any sorting column (edited_on, application, reference, module, ...) the current filter now stays active, were previously you lost your current selection. 
You did this on some other places too.