Restructured app, updated code review observations

Still pending changes in list_view:
- sub levels allowed
- some documentation updates
Still pending changes in file_properties:
- documentation and code review corrections

Both will be sent by Gaby
Added comments and examples on list_view and item_operations methods.
Changed the file_operations name to item_operations file name.
Changed the ListView type from QListView to QTableView.
Defined a new method to set the QTableView style and columns displayed.
Fixed tab problem with action bar.

Fixed problem where tab included action bar buttons. Is is now working
properly and selecting only panels
Moved QFileSystemModel to Base_Item

In order to separate logic from UI, model will be defined in BaseItem
class to interact with proper methods
Doc Updates for BaseItem
Propagate changes to panel_model(BaseItem)

change functions to propagate path changes to BaseItem, this way
functions will run in proper paths
Replaced with item_operations
Added the documentation missing, except on methods related to
panel_toolbar and update path.
Moved the UI to the corresponding place from ItemOperations.
Reorganized the ItemOperations methods.
Added remaining changes from code review
Merge pull request #13 from lisandro-montano/feature-items-restructure

Restructured app, updated code review observations