Fix for battery device icon update

[quozl]

(this is the updated commit message from 67ffe98)

Frame battery device is static on Ubuntu 16.04, with UPower 0.99.4.

The 'changed' signal is not sent. UPower had deprecated 'changed', replacing with 'notify' for all properties, or on per-property basis.

• switch to UPowerGlib GObject introspection API,
• continue to use 'changed' signals on older UPower, for compatibility with OLPC OS 13.2.7, Fedora 18, UPower 0.9.19,
• switch to 'notify' signals if they are present,
• continue to use D-Bus API to get device path, because the older UPower fails on get_devices method, and doesn't have get_display_device,
• ignore bursts of notify signals; reduces frame icon redraws,
• fixes https://bugs.sugarlabs.org/ticket/4781.

Tested with OLPC OS 13.2.7 on XO-4, and Ubuntu 16.04 on commodity hardware.

Requires gir1.2-upowerglib-1.0 on Ubuntu 16.04.

[quozl]

Rewrote patch;

• avoid checking for get_display_device, always use the D-Bus enumerate devices result,
• split out the connecting of signals into a private method,
• check the signals using GObject.signal_list_names,
• include test results.

[samdroid-apps]

Couldn't we just use the UPowerGlib constants directly? Find and replace is good :)

[quozl]

• use UPowerGlib constants directly instead of making our own based on them, reported by @samdroid-apps,
• confirmed _UP_STATE* were not used outside module.