make file operations accessible from text editor

[brainchild0]

The discussion in #6 lead me to realize that inclusion of file management features in the text app would improve usability in a number of ways.

Currently, the user may access some file information in the sidebar, but not the basic operations, such as renaming, moving, or copying, that are normally accessed using the ellipsis icon in the file app.

Changing the file name especially would seem to be a common task that is more conveniently accessible without leaving the editor to return to the file view.

[juliushaertl]

This is something that once the viewer app has implemented it in nextcloud/viewer#15 it will automatically become available in text.