filters: Submit the form when <Enter> is pressed on a closed submitter field

filters: Submit the form when <Enter> is pressed on a closed submitter field

<Enter> when an element of the form is focused should submit the form.
Before this patch, when <Enter> was pressed on the submitter button,
nothing would happen.

So, we tweak our <Enter> handling on the submitter input a bit more:
<Enter> can be used to select the submitter from the autocompletion
list, but, when pressed a second time (ie. when the autocompletion
option has be selected and the dropdown isn't shown), we submit the
form.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>