Trash folder fixes and Notice Bar

[vkbo]

This PR adds a read only flag to documents that the document editor class respects. This is in particular used to prevent the editing of files in the trash folder.

The PR also ensures that new items cannot be added to the trash folder and the orphaned files folder.

To notify the user that the file is read only, a notice bar will pop up at the top of the editor letting them know the file is read only. The notice can be dismissed.

These changes resolve issue #105

Also in this PR:

• Split the source files in the ever growing gui folder into a few sub folders.
• Slightly adjusted the margins of the main window such that all content between menu and status bar has a 4 pixel margin, and the margin of the treeview pane has been removed.