DisplayDevice is a "special" device object that represent what a widget should
display, however it does *not* give information about Line Power. E.g. when a
laptop has no battery but is plugged on the mains, DisplayDevice says that no
device should be displayed and the icon is set to "battery-missing" (which is
in fact true).
However, if we are working, it means that we must be getting power from
somewhere, so let's display something in the tooltip anyway.
Command line example
$ upower -i /org/freedesktop/UPower/devices/DisplayDevice
power supply: no
updated: Sun 05 Nov 2017 04:19:48 PM GMT (415 seconds ago)
has history: no
has statistics: no
unknown
warning-level: none
icon-name: 'battery-missing-symbolic'
See also https://upower.freedesktop.org/docs/UPower.html#UPower.GetDisplayDevice
Instead of using Awesome's DBus api, use directly the on_properties_changed
method on the device object. This change removes a lot of boilerplate code and
makes the module much simpler!
- remove section about installing from source: this requires some juggling and
symlinking, so I will leave it to people who *really* want to do it
- explain how to export the local luarocks path when installint locally with luarocks
Whenever `ctx:iteration()` is called, when the PropertiesChanges signal is
emitted, awesome reports a problem with data left in the Lua stack:
```
2017-04-11 07:08:52 W: awesome: a_glib_poll:372: Something was left on the Lua stack, this is a bug!
-------- Lua stack dump ---------
3: nil
2: userdata #16 0x2251a78
1: userdata #320 0x27c5408
------- Lua stack dump end ------
```
This may eventually result in crashes or freezes. To fix the problem, don't use
the GLib context, instead:
- grab all properties from the proxy and copy them over into a table
- use that table as the data for the widget
- update the table when the PropertiesChanged signal is emitted
Stefano Mazzucco