Project manager for gedit

For this plugin a project is just a directory path. A list of last opened
files is stored in the user config directory. In the project directory itself
no data is stored or changed by the plugin. In the sidebar are two lists:
* Open Projects: projects with at least one open file
* All projects: project directories that are known to the plugin,
projects that contain subprojects are shown in a tree structure.

In the context menu you can perform some actions, including:
* Open Project: restore all recently opened files of a project
* View Project Folder: the project folder is opened with the default program
* Close Project: all project files are closed (the file list is preserved)
* Add Folder, remove Folder: specify which folders are known as a project
* Find Subprojects: search w