Allow MRU file list length to be set to 0 to disable it

Since the rest of the code is already prepared for it, setting mru_length
to 0 just disables the recent files menu.
Fix closing of the "missing file" infobar

The issue was introduced in b646424ddb715382a937d233a75bc684c22e18ec as
it removed the default handler that used to destroy the infobar.

Now, the infobar is properly closed, but only if the action succeeded.

Closes #375.