Indicator for parseable facts in gui

[ederag]

With merged PR #461 and pending #466,
it will be possible to enter anything in the gui separate fields.
The rationale was given in this comment.

It will be possible to enter a fact that can not be parsed,
especially until #465 is solved.
This means that the quick cmdline entry can not be used any longer to edit such a fact.

The recommended way is to stick to facts that can be manipulated through the parser
(both command line and gui cmdline),
so there should be an indicator for safe syntax.

Entries in the gui cmdline can be easily checked:
A simple look at the separate fields that are updated as writing.

Entry in a separate field updates the cmdline text
(which is a single string representation of the fact),
so this is easily check too.

The missing part is whether parsing this cmdline text would yield
the same Fact as the separate fields.

Probably a green light if this is the case,
and orange if the round trip does not work.
Probably not turning to red because that would be obnoxious to people
accepting to loose the ease of the quick cmdline entry.

Of course a change in background color as suggested by @mwilck at the end of #461 (comment)
would be better, but having a single indicator is much simpler.

[ederag]

No support from anyone so, following #461 (comment) and #461 (comment),
this proposition is abandoned in favor of the stricter #476 PR.