Simplify/optimize battery util on linux

[rrrapha]

Instead of looping over a copy of the device objects, get the appropriate device directly via up_client_get_display_device().
Checking is-present makes sure that hot pluggable batteries are handled correctly.

The upower client is only created once in the class constructor.
(There was a comment about this in the code, but I could not reproduce the issue)

Note that this removes support for upower < 0.99.0. (It should still work on Xenial)

[uklotzde]

The uncontrollable side-effects caused by redefining symbols in the header file are not acceptable. Revert those changes and replace with a forward declaration.

[uklotzde]

I'm not convinced that the code gets easier to follow by using deeply nested if blocks. Safely initializing the members with invalid/unknown default values and returning as soon as possible when you are not able to read the required values is simpler and more robust.

[uklotzde]

Much easier to read now. Just some minor remarks.

[uklotzde]

The void* member is the correct solution here.

[uklotzde]

Thank you for taking care of these little performance details that easily slip through unnoticed!

LGTM

[rrrapha]

I discovered an interesting side effect of the 'create the upower client once' change.
When I stop the dbus daemon while mixxx is running, mixxx receives a SIGTERM signal and terminates immediately.
This is not ideal IMO, but I don't know how to 'fix' it ..
@uklotzde Do you think we can live with that?

[uklotzde]

Stopping the dbus service kills my whole desktop. When stopping the upower service I don't experience any issues.