linkmap: Fix rendering when textview and linkmap are offset
Prompt user for action when file changes externally (closes bgo#460201)

When we get a notification that a file has changed externally, we now
check mtimes, maintaining both the last mtime we saw (to prevent
duplicate notifications) and the mtime at which we loaded the file.

This doesn't yet prompt the user to overwrite a changed file; they've
already been prompted to reload, but we should still prompt for the
overwrite.
meldbuffer: New-style property declaration
meldbuffer: Make the reset_buffer() call not recreate MeldBufferData

With this change, there is always one MeldBufferData per MeldBuffer,
which makes it much easier to reason about connecting signals to it.
filediff: Since MeldBufferData now isn't recreated, connect at creation
filediff: Prompt user to overwrite a file that has changed since load