Support opening a folder

[joeha480]

Support opening a folder in addition to opening a single file. When opening a folder, there should be a panel where the file tree can be accessed. It should be possible to open files in the tree for editing (if the file format is supported). It should also be possible to add, rename and delete files in the tree and create, rename and delete folders. It should also be possible to move files within the structure.

The folder should be watched so that external changes can be detected.

This functionality is a first step towards editing of zip-containers (such as an epub-file).