Add some custom properties to elements for modding

As a modder I am missing unused read / write properties to save some stuff or use it for intermediate changes or for formulas. So I really want a bunch of CustomProperties added to

- MajorEmpire / Empire

- Cities / Settlement

- District

- Territory

- Unit

- Army / Unit Collection / Squadron

like it was done with the additional LuxuryCounts, which made it possible for uncle to do his mod.

Examples of why:

- I made a pop growth stop mod. This works fine with outposts but due the ability of having a pop consumption multiplier I cannot use a formula to easily foresee next pop requirement in cities. I currently made the same formula several times and added validation for checking which to use. With a custom property I would be able to save the "future" pop consumption in a property which can also be influenced by techs.

- In my Revised Unit Combat, the Aircraft Carrier CS is dependent on the squadron CS. Since I cannot add a value to the army which basically counts the number of carriers I had to use an event system, which has limitations.

I would also like to count Capital Ships and Screening Ships for purpose of cs malus or not by certain attack types

- On Settlement / City this would allow to not count the workplaces but certain types of districts (EQ, Maker, Farmer, Artisan, etc.) which does stuff (influence district costs for example or other modifiers which relies on them)

- On Empire this can be used for a lot of stuff... my "Military Supplies" could use it too...

There are many instances where I really would have liked having this as a counter or use it to influence certains stuff or use it as a modifier which is then used as a multiplicator somewhere else.

Or let us be able to add them by mod tools ourselves :)