FlightGear - Flight Simulator FGAddon

PC-12: feed the Hobbs timer.

* Nasal/hobbs.nas: new.
* pc12-base.xml: load it.

Citation version 3.4.2: add a Hobbs engine-time meter.

* Models/Instruments/panel-layout1.xml: add a Hobbs meter, not exactly at the same position
as in [1], but close enough. I'd rather not sacrifice the copilot's AoA and OAT gauges.
* Nasal/citation2.nas (JetEngine): add a timer to each engine, measuring how long
/engines/engine[*]/running is true.
(hobbs_timer): new. Feed the Hobbs timer with the maximum of the two values.
(update_systems): call hobbs_timer.update ().
* Nasal/save-state.nas (citation_save_general): save and restore the engine running times.

[1] http://pureflite.com/sites/default/files/forsale/20.jpg

J3Cub: Allow tailgear and weight in all configurations

new helpers/classes

Fix typo in Rodolfo's firstname, sorry!

Add-ons: update addon-metadata.xml files following FG commit c3b1442546

Concerning the "GNU GPL version 2 or later" set for ATCChatter and its
authors: this is based on 1) the LICENSE file and 2) Torsten's message
on flightgear-devel from Thu, 07 Dec 2017
(<https://sourceforge.net/p/flightgear/mailman/message/36150299/>) which
said that the add-on was previously in FGData, and thus a priori
following the project licensing policy (GPLv2+ for FGData).

For SpokenATC and SpokenGCA, this was agreed by Rodolfo Leibner using
private messages on the FlightGear forum.

PC-12: add a standby altimeter, artifical horizon and Hobbs time counter on the pilot side.

pc12-yasim.xml: bug fix: left and right main landing gears were swapped, affecting only
differential braking.

Reference: http://cbaviation.com/wp-content/uploads/2017/06/Cockpit-3.jpg

PC-12: delete the angle-of-attack indicators from the cockpit.

The real aircraft doesn't have them and they didn't work properly anyway.

PC-12: delete unused instrument files, continuing r3164.

PC-12: improve realism of the cockpit.

* Models/Interior/Panel/Instruments/{KT76A,chrono}: delete.
* Models/Interior/Panel/Instruments/ki206: copy from the DR400-dauphin. Modify to use nav2
instead of nav1. (Note: this is because the stock KI206 in $FG_DATA/Aircraft/Instruments-3d
is not satisfactory).
* Models/Interior/Panel/panel.xml: use the stock M877 clock and place it like on [1].
Swap the positions of COM1/NAV1 (now on top) and COM2/NAV2 (now on the bottom) in the radio stack.
Use the stock KT76A from $FG_DATA/Aircraft/Instruments-3d.
Add a KI206 instrument for NAV2, where the (oversized) clock was previously, like on [1].
* pc12-base.xml: load the nasal module for the M877 clock.

[1] http://cbaviation.com/wp-content/uploads/2017/06/Cockpit-3.jpg