Search notes context menu rework

Here are some improvements to context menus in Gnote Search All Notes dialog.

What's wrong with existing ones?

• You can create new notebook via context menu, but not new note, which makes the two panes inconsistent


• When you have a lot of notebooks so there's no empty space to right click on, it's not possible to create new notebook

Improvements

• 
  Added New Note item to context menu in notes list:
  
  
  


• Right click anywhere in notes pane now brings context menu, as New Note item is always available


• 
  When a notebook is selected, creating new note from context menu adds it to selected notebook
  
  
  


• 
  Added new notebook item to notebook context menu:
  
  


• Empty space context menu was removed from notebooks pane, there's only one context menu for entire pane now


• In both panes the items above separators apply to selection, items below apply to entire pane

In future versions of Gnote the Search All Notes window will be the main application window.