Add settings menu for folder of index files

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.

Locate the settings UI and the code that stores index-cache files for the currently opened DLT file; no specific files or tests are named in the issue. Define how the setting selects local versus centralized storage, then verify that index files are read and written in the selected folder.

Written by the indexing model from the issue text.