add dropdown to "add language resource" button

to enhance usability, please

• change the "[+] Add language resource" button to a menu button, containing the list of available resources (as in the screenshot) as menu items

• after choosing a menu item, the usual popup window for language resource creation should open
• the chosen category like TermCollection etc is already pre-selected in the existing dropdown then
• the cursor should start in the field "name" of the resource
• Make the file selection field as drop down target for files (see [+]Add Project Button, there it works already)