Category versus Tag

[theAkito]

Is your feature request related to a problem? Please describe.
I migrated from a different app and "categories" were "tags" there. So, you could check and uncheck all the tags you wanted, to get exactly the entries you need to see. However, with "categories" in this app, I can only choose a single one at a time.

Describe the solution you'd like
Either enable selecting and de-selecting multiple categories at once (easy, but not good) or add tags additionally and make the user decide himself, how he organises his entries (bit more work, but ideal solution). A combination of both, categories and tags, would allow for more advanced entry ordering.

Additional context
Definitely very important to fix #514 and implicitly apply it to the new tags feature, as well.