Set an icon for the "Overview" button

Use view-grid-symbolic icon for "Overview".
Action identifier for administrative privileges

Add a method that returns the PolicyKit action identifier for modules
that requires administrative privileges.

Issue: #11
Administrative privileges

Handle administrative privileges for the modules that require it.

Issue: #11
Change search line edit visibile with action

We can't directly show or hide the line edit, we need to do it with the
action.
Set attributes according to administrative privileges status

The action now has different label, icon and tooltip according to the
administrative privileges status.

Issue: #11
Show text besides toolbar button icons except for the overview button

Set toolbar button style to show text besides icons.
The overview button doesn't have text anymore and it's created with a
QToolButton instead of a QAction otherwise it would appear larger than
other buttons because it makes room for the text (which is not
specified). The only action with text is the Lock/Unlock button for
administrative privileges.
User account creation and deletion

Implement user account creation and deletion.
User account preflet requires administrative privileges

It requires administrative privileges to work.
Make sure we pick up the right PolkitQt-1 library

There could be another library available (one that has not ported to Qt
5 yet) so we need to make sure we pick up the right one.