[RAPPS] Centralize RAPPS exposed names: registry & user local settings (#4214)

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

- Change the name of the application database sub-directory from
  "rapps\rapps" to "RApps\appdb".

- Adjust the documentation to reflect this.

- Simplify CConfigParser constructor by removing GetINIFullPath().

base/applications/rapps/settingsdlg.cpp

@@ -246,7 +246,7 @@ SettingsDlgProc(HWND hDlg, UINT Msg, WPARAM wParam, LPARAM lParam)
                         CStringW szMsgText;
                         szMsgText.LoadStringW(IDS_URL_INVALID);
 
-                        MessageBoxW(hDlg, szMsgText, NULL, 0);
+                        MessageBoxW(hDlg, szMsgText, NULL, MB_OK);
                         SetFocus(GetDlgItem(hDlg, IDC_SOURCE_URL));
                         break;
                     }