[RAPPS] Centralize the names of the RAPPS' registry setting sub-key, and user local settings & offline apps database sub-directories.

[HBelusca]

Purpose

The primary goal is the replace (in user's local AppData) rapps\rapps, which looks a bit ridiculous and could appear as a bug of RAPPS (why creating two subdirectories with the same name??), by instead RAPPS\AppsDb, which (to my opinion) looks more decent and tells exactly what it is for -- the offline apps database. This would also allow for future sub-directories into the RAPPS settings folder.

And also I take the opportunity to consolidate in one single place the defines for these directory names.

Proposed changes

• Change the name of the application database sub-directory from "rapps\rapps" to "RAPPS\AppsDb".
• Adjust the documentation to reflect this.
• Simplify GetINIFullPath() in the process.

NOTE: In case this is accepted, https://reactos.org/wiki/RAPPS will need minor adjustments as well (following the README update).