filechooserentry: Don't accept completion on activate

In the FileChooserDialog, the FileChooserEntry accepts the current
autocompletion when "activate" is signaled (when the enter key is
pressed) and begins the next autocompletion.  The FileChooserDialog is
activated by the same signal so this can result in an unexpected file or
folder being selected.

Add a function to gtkentrycompletion.c to optionally disable the accept
completion on activate behavior and have FileChooserEntry call it.

GNOME/gtk#42
2 jobs for filechooserentry-activate in 18 minutes and 8 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #16094
build

00:18:08

failed #16060
build

00:00:39