Don't show files which don't exist in the recents list

It doesn't make much sense to keep non-existing files in the recent file
and project list. Renaming a project directory for instance leaves the old
project paths in the recent project list forever.
Disable current project in recent project files

Nobody wants to open the project which is already open. This however
happens quite often by accident (which closes the project and reloads
it again).

(Keeping recent files in the list makes sense for open files because it
just switches to right tab.)