Some fixes to backlight module

[thiagokokada]

This PR brings 2 fixes to backlight module:

• It fixes the following exception when the user does not have a backlight in the system (i.e. Desktop) and tries to adjust the brightness:

2019-05-27 23:05:53,754 [CRITICAL][i3pystatus.backlight.Backlight 175] Exception while processing button callback: AttributeError("'Backlight' object has no attribute 'has_xbacklight'")
2019-05-27 23:05:53,754 [CRITICAL][i3pystatus.backlight.Backlight 176] Traceback (most recent call last):
  File "/nix/store/v0km55axl2452hnsazbc1y8k0zdmcsbv-i3pystatus-unstable-2019-02-10/lib/python3.7/site-packages/i3pystatus/core/modules.py", line 162, in __button_callback_handler
    call_callback(getattr(self, cb), *args, **kwargs)
  File "/nix/store/v0km55axl2452hnsazbc1y8k0zdmcsbv-i3pystatus-unstable-2019-02-10/lib/python3.7/site-packages/i3pystatus/core/modules.py", line 133, in call_callback
    cb(*args, **kwargs)
  File "/nix/store/v0km55axl2452hnsazbc1y8k0zdmcsbv-i3pystatus-unstable-2019-02-10/lib/python3.7/site-packages/i3pystatus/backlight.py", line 93, in darker
    if self.has_xbacklight:
AttributeError: 'Backlight' object has no attribute 'has_xbacklight'

• It allows the user to set format_no_backlight to empty string and hide backlight module in case the system does not have a backlight (i.e.:, again, in a Desktop). If you set to an empty string nowadays it results the same output as the default one, resulting in something like Brightness: -1 output.

[thiagokokada]

Superseded by PR: #749.