[Bug] demo location uses a hard-coded path instead of `GISDBASE`?

[infrastation]

Describe the bug
The GUI disregards an existing non-default database directory when automatically creating and activating a demo location.

To Reproduce

1. install GRASS from the master branch one or two months ago (it could be related to commit 28876fe)
2. configure a non-default GISDBASE in ~/.grass7/rc and fill it with a few locations and layers
3. exit GRASS, update to the current master branch and start it again
4. upon start the GUI will create a new database in ~/grassdata with a location named world_latlong_wgs84 and a vector layer, it will also switch to the new mapset

Expected behavior
It would seem more logical to create the demo location in the current database, and to prompt the user before doing so. Perhaps that's how it works when GISDBASE points to the default directory, but when it is different, the behaviour becomes inconsistent.

[lindakarlovska]

Please, see #1400. Here is explained how the suggested startup is gonna look like.
As @HuidaeCho suggested, I will probably make another variable in rc called "default_gisdbase" which could be manually set up by a user.