[tools] simple poller to monitor statefs data

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[poll] protect dumb clients from infinite polling

If client tries to poll continuous file poll will not return error, some
libraries does not distinguish between poll error event and data
event (like QSocketNotifier). Also BasicTextFile poll event is emited
only once because content of the file is constant

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>