ENH: Interface Improvements

[bradyzp]

List of interface improvements that need to be accomplished at some point, for tracking purposes:

• [x] Icons for Data Files based on type (GPS/Gravity)
• [x] Ability to delete Flights, Data files etc. from the Tree View
• [x] Display Data Files in Tree View in a informative way e.g. : Filepath
• [ ] Add ability to show more detailed information for items via context menu (similar to Flight info menu)
• [ ] Context menu for Data Channels to remove all/remove all from specific plot
• [x] Fix TreeView style sheet to display arrows on all expandable menus
• [ ] Improve loading splash-screen functionality: enable button to clear recent. Option to sort by date.

[bradyzp]

Adding to this list of things to do in the GUI department - post merge of PR #69

• [x] Implement cross-platform application state management using QSettings (track recent projects, window size, open flights etc)) [in progress]
• [ ] Use Qt stylesheets to control various interface styling (see issue #70 ) - possibly add ability for user to select font type/size etc through dialog menu (and remember the setting using QSettings).
• [ ] Improve the QMenu system used for the project tree, possibly enabling inheritance of menu items from a parent.
• [x] Flatten the project tree, remove the Datasets folder; DataSet will become a direct child of its Flight. Sensors folder may also be removed, sensor links can be added to a DataSet. (resolved by #79 )
• [x] Enable loading/opening of multiple projects within the main window/project tree - this may require some project updates but I think is almost doable. (resolved by #76 )
• [x] DataSet Segments need the ability to be deleted by the user via GUI action.
• [ ] Add a dialog/menu action that allows a user to create or edit a DataSegment (as opposed to click-dragging on the plot surface)
• [ ] Add a dialog that allows the user to inspect an imported Data File (i.e. Gravity/Trajectory data), displaying statistics ala DataFrame.describe(), and allowing the user to browse the raw data using a QTableView (possibly enabling editing).