Use "x-office-presentation" icon name

And use the icon-name property in MenuToolAction instead of stock-id.
latex menu: remove dead code
FileBrowser: use icon-name instead of stock-id
Structure: use icon-name instead of stock-id
Use image-x-generic icon-name instead of stock-id

So no need to register some icon-names as stock-ids.
Build Tools: use icon-name instead of stock-id
Side panel: use icon-name instead of stock-id
Symbols: use icon-name instead of stock-id
Don't use gtk_image_new_from_stock() (deprecated)
Don't use gtk_widget_render_icon_pixbuf() (deprecated)
icons: follow freedesktop icon theme specification
icons: move stock-icons to data/icons/

The icons available in several sizes are no longer duplicated with
suffixes like "-size16" and "-size24", they are just put in the right
directories.