Add settings menu for folder of index files

[alexmucde]

Since v2.23.0 the index file for the index cache are stored in a subfolder index of the current openned dlt file. In v2.22.0 there was a single directory configurable, where all index files were stored. Both solutions have advantaged and disadvantages. In the settings an option should be added, where the user can define, where index files are stored, locally or centrally in an folder.