Add "Upload file" and "Create file" actions to Sandboxes context menu

Currently, it is possible to edit existing files and create a new directory, but there is no way to easily create an empty file in the Sandboxes module. I think there could be an action in the context menu of the project tree that would ask the user for the name of the file and create an empty file in the selected directory.

Similarly, when uploading one file to a selected directory, one has to create a ZIP file with the correct directory structure. There could be an action that would allow the user to select a file and upload it to the selected directory.

The actions should handle the case when the target file already exists.

Optionally, enable drag&drop support for file upload, if it's not difficult.