With this patch we move the codebase to Rust-2018.
The diff was generated by executing
cargo fix --all --all-features --edition
on the codebase.
Signed-off-by: Matthias Beyer <mail@beyermatthias.de>
This patch rewrites the whole libimagcategory and brings it to a nice
standard (the code before was rather messy).
Now, categories are represented by an entry in the store and all entries
which have this category are linked to that entry.
