speeding up the start through Checkbox for DIS-abling read all library-examples

to me it is really annoying that the Arduino-IDE needs so much time for startup.
So yeserday I [asked in the forum](https://forum.arduino.cc/index.php?topic=713728) how to speed up the start.

One tip is:
And if you have a billion libraries installed ( :D ), clean it out. The IDE reads all of them to populate the examples.

So my suggestion is to add a checkbox (or at least a preference-item `scan_all_libraries_for_examples=false`

best regards Stefan

No files or tests are named. Start by locating the startup path that scans installed libraries to populate examples and the existing preference handling; determine where a checkbox or scan_all_libraries_for_examples setting belongs. Done means startup can skip the library scan through a user-visible setting while preserving the existing behavior when enabled.

Written by the indexing model from the issue text.