SOLID UI: refactor file-browsing code built upon `basePath`

[cdawei]

The basePath concept (introduced in a previous version of solidpod for file IO, it has been removed in recent releases of solidpod) is heavily involved in the file-browsing codebase, and this leads to a leading forward slash / in the relative path of a file to be read, e.g., /myapp/encryption/ind-keys.ttl.

This format triggers an error when extracting data from the turtle content as

• There will be a double // in the file URL
• The subject of triples in .ttl is the file URL
• readPod() assumes the above two are identical when decrypting content, but they are not in this case.

A temporary workaround (removing the leading forward slash in a relative path if it presents) has been implemented in solidui but a better solution is needed to fully resolve this issue, e.g., refactor the file-browsing code to use PathType instead of basePath.