Detecting whether Krita comes from a store

[Halla Rempt]

Hi,

Now we've got the begging banner, we need to make sure that people who already have paid for Krita, only see the banner if we're running a special campaign. So, for each store we need to have a way to figure out if it's a store version of Krita. (I forgot what the rules were on the Android Store, but we still have the small donation link, so the support request might be fine?)

I've added an empty KisApplication::isStoreApplication method, and that needs fleshing out. 

So, please, for each of these stores, tell me what diagnistics will work:

* Windows Store

(Check where the executable is? An environment variable? The location of the user data?)

* Steam Store

** Windows

** Linux

* MacOS Store


For the Epic Store, I'll try to figure it out myself, but that's one of the smallest anyway.

We really want to have this done before the second beta, so please give this a bit of priority...

Halla