Allow to select costum data folder location and auto add zites to the client in the folder

**Is your feature request related to a problem? Please describe.**

If the user downloaded many zite it's not possible to the program auto add the sites from the data folder. The user must manually add all zites with pasting the zite URL in the browser to add it to the ZeroNet client. There is no possibility to select a costum "data" folder location. This way not possible if the user use different OS like Linux and Windows to select the data folder location and use the same setup in different OS. Or share in a shared storage in different computers the same content.

**Describe the solution you'd like**

A button to allow to select costum "data folder" location if the user does not want to use the program folder. And automatically add all zite to the client what include the folder.

The issue names no files, tests, or entry points. Start by locating the client's current data-folder configuration and zite import path, then define cross-platform folder selection and automatic discovery behavior. Done should include verified behavior for selecting a shared or alternate data folder and adding its zites automatically.

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