Problem title: File selection in batch mode

Description:

When trying to batch edit a project’s files, all files for all languages appear in a selection window and can be ticked or unticked. There is, however, no option to “Select all” or “Unselect all” (pretty basic), which makes it extremely frustrating and time-consuming to select a given scope of files in a project with both multiple languages and multiple files.

Expected behavior:

There should be a “Select all” and “Unselect all” option. Additionally, the user should be able to select files by language (all files) or by project file (all languages). In other software such as Passolo this function exists.

Screenshot: