balooctl monitor: Wait for dbus interface

Summary:
If baloo is not running, monitor will wait for its start.
This will simplify baloo diagnostics.

Test Plan:

$ balooctl stop
$ balooctl monitor

Without patch:

Baloo is not running
Press ctrl+c to exit monitor

With patch:

Press ctrl+c to exit monitor
Waiting for Baloo to start

In different console: balooctl start
Without patch: Nothing happens
With patch:

Baloo is running
QDBusConnection: name 'org.kde.baloo' had owner '' but we thought it was ':1.238'

Change a file in indexed folder. E.g.

$ echo otto >> ~/Documents/otto.csv

Without patch: nothing happens.
With patch:

Indexing: ~/Documents/otto.csv: Ok`

In different console: balooctl stop
With and without patch:

Baloo died
$

Reviewers: dfaure, alexeymin

Reviewed By: alexeymin

Subscribers: cfeck, alexeymin, Frameworks

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D10578