Some changes and fixes to projects.

A few changes to better support symbolic links as well as directories not under VCS. If the target of the symbolic link changes, it should change nothing for the project, provided that it is closed and reopened. Opened files should still point to the path with the symbolic link.

• Fix exclude_patterns in the example.
• Remove canonization of paths so that symbolic links are left intact.
• Document exclude_patterns and new hidden option.

For directories not under version control:

• Will now follow symbolic links. These will be treated like normal directories transparently. Removal of canonization of paths is necessary for this.
• Add an option to include or exclude hidden files.
• Fetch files and directories, so that empty directories can be listed.

Related change: frameworks/ktexteditor!448 (merged)