scan-registers: determine available notifications

00 is documented in HID++ 1.0 specification, it should be safe to set the 00
register. As another program may have written the notifications register
already and since enabling notifications has side-effects such as "disabling"
certaing functions, restore the flags after reading out available notifs.
Merge pull request #41 from Lekensteyn/tool-updates

scan-registers: determine available notifications
enable notifications for peripherals; fixes #27
only enable certain notification flags on devices

Also added some documentation to the notification flags, where possible.
minor readme update
only notify on connected devices at start-up
pop up a desktop notification if the battery is below 5%
depend on gnome-icon-theme or oxygen-icon-theme

It's necessary to make sure all the standard icons are available.
don't crash when failing to load the icon mask
version 0.8.8.1
about window tweak
hid10: document notifications better

See [registers.txt][1] for 'keyboard_backlight'. The behavior of
keyboard_sleep_raw and keyboard_multimedia_raw is described at
[keyboard.txt][2].

 [1]: https://git.lekensteyn.nl/ltunify/tree/registers.txt
 [2]: https://git.lekensteyn.nl/ltunify/tree/keyboard.txt
hid10: Formatting and re-order NOTIFICATION_FLAGs

Align values for easier reading the numeric values of NOTIFICATION_FLAGs.  To
maintain consistency in the ordering of the values, swap keyboard_backlight with
keyboard_present.
hid10: update flags description for notifications

The Logitech HID++ 1.0 documentation actually names the fields for devices and
receiver. Clarify that and explain why enabling all of the bits is a bad idea.
Merge pull request #48 from Lekensteyn/doc-updates

Doc updates for HID++ 1.0 notifications
Merge remote-tracking branch 'origin/master'