up_client_new() is transfer full and failable constructor.

The callback for the keyboard brightness keys has its functionality restored,
with clarifying comments added to avoid similar mistakes being made by future
refactoring efforts.

The functions which return the next brightness level to be used are refactored
to ensure the value never decreases below 0 or above the maximum brightness.
Calls to these functions are updated accordingly.

Fixes various critical especially when switching from plugged to
battery.

Make the string more consistent with what we have in the plugin's menu
and what the name of the option suggests (first percentage, then
remaining time).
See also !17 and #128

Simon Steinbeiss authored 4 years ago and Simon Steinbeiß committed 4 years ago

Previously we relied entirely on UPower, now we also check if the icons
used by Gnome are available (battery-level-*).

This commit introduces two new options:
1) Configuring the amount of brightness steps
2) Whether to distribute the steps in a linear or exponential way

Both options are presented in the settings dialog and affect the
brightness keys and the panel plugin.

This hopefully makes the icon stand out a little more.

This means we're now consistent with other panel plugins like pulseaudio or
notifyd.

The label will only be hidden if the display device, i.e. the one
showing up in the panel is the desktop itself.

Unfortunately UPower does not differentiate between desktop and laptop
systems so we decide that it's a desktop if it:
 * is not a UPS
 * does not have a battery
 * does not have a lid
In this case we show the "ac-adapter" icon for the systray and the
"ac-adapter-symbolic" icon for the Xfce Panel Plugin.

Before the "battery-missing" icon was shown, which is what UPower
reports for the display device.

DEVICE_STATE_UKNOWN means that there is no battery available.

Signed-off-by: Christian Hesse <mail@eworm.de>