The story
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As a GThumb user who uses catalogues to keep track of the good shots in a given photoshoot
I want to be able to click an "Add to last catalogue" button
So that I don't have to dig through right-click menus for 100+ images. 


Acceptance criteria
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* An "Add to last catalogue" button is present in the GThumb toolbar in the thumbnail and image views. 
* When clicked, the button adds the current image (image view) or selected files (thumbnail view) to the 
  most recently used catalogue.
* The hover tooltip should be "Add image(s) to <catalogue name>". 
* The most recently used catalogue is the catalogue most recently created or the catalogue that most 
  recently had files added to it, whichever is newer. 
* If Gthumb doesn't know what the most recently used catalogue is, the button should be disabled. 
* If the most recently used catalogue is deleted, the button should be disabled. 
* The button should only be visible if the catalogues extension is enabled.