Selectable Hardware folder for Boards Manager

Using the new IDE I had "problems" in finding the hardware folder where the core is installed.

Talking about this I think that could be nice to add an "open hardware folder" somewhere in the IDE in order to have a quick link to that folder. This is useful when someone has to search for something in the core.

Another way could be to make it selectionable by the user as for the sketchbook because in this way for example as i did fo my sketchbook i can place it on my dropbox folder so i have my sketches every where. Doing it in this way the user can have the boards installed on every machine without downloading the core every time.

However i also think that a little improvement could also be to choose the sketchbook (and hardware folder) location at the first Arduino IDE start. In this way a (maybe) unuseful folder isn't created.

Start by tracing how the Arduino IDE stores the sketchbook and installed Boards Manager hardware folders. Compare the requested choices—an open-folder shortcut, a user-selectable hardware location, and first-start configuration—and establish one scoped behavior before identifying the affected settings and UI entry points. Done means the selected location is used consistently and existing installations remain usable.

Written by the indexing model from the issue text.