fix assert failure, when a strange shortcut has been set for a deleted menu item

git-svn-id: http://svn.code.sf.net/p/texstudio/code@4139 92cce386-e96d-4106-809f-596c673a3fd8
remove deleted files from recent file/project list, allow to select multiple recent files/projects in the recent file dialog

git-svn-id: http://svn.code.sf.net/p/texstudio/code@4140 92cce386-e96d-4106-809f-596c673a3fd8