Refactor File Model + Sidebar Folder Functionality

This PR encompasses a multitude of changes that stemmed from the refactoring of the file model. The following is an overview of the changes that were made:

• Created a FileNode that is essentially a wrapper for a File with our own functions.
• Most of the logic from FileModel was moved into FileNode. For example, FileNode is responsible for creating child files/folders, loading, saving, exporting and more.
• Got rid of idea of keeping our FileNodes in an array and using an index to keep track of which file is selected. Instead, we now work directly with the FileNode
  • Rationale: This idea gets messy when we dynamically add notes to our file structure. Also, it does not translate well to a folder-like structure
• Used TreeView as a framework to build the UI of the sidebar.
• Integrated the refactored models with all features (unless I missed some when going through the app). Some include: exporting to pdf, saving files, loading files, menu bar actions, etc.

Some features that were added for the sidebar are:

• Got adding new files to specific folders working (the buttons on the top of the sidebar will default to adding new notes for the selected folder - it does nothing if the current selected file is not a folder)
• Added functionality for creating new folders (again only works if the selected file is a folder - doesn't makes sense to add a folder to a file
• Added right-click context menus for adding new files/folders to whatever the user specifies

I will be properly adding styling to the sidebar in the next ticket.