RFC: replace union and intersection symbols of filters

[zisoft]

This PR replaces the mathematical union and intersection symbols of the color label filter with (IMO) easier to understand & for AND and || for OR.

I have marked this as RFC because this will for sure not meet everyone's taste.

fixes #15152

[ralfbrown]

I'm not sure non-programmers would understand the || any better than the mathematical symbol....

[parafin]

I even see a problem with & symbol - while everyone should know, that it means “and”, only in boolean logic “and” means intersection. In normal language it can mean both union and intersection (e.g. in this sentence it’s union). I would even say it’s more common for “and” to mean union.

Maybe try to come up with icons, that would symbolise phrases “narrow down” and “expand” instead.

[pehar1]

The symbols currently used describe precisely which operation is meant. In my opinion, they should not be replaced by "&" and "||".

The ampersand ("&" sign) is a ligature, which is used as a substitute for the “and” in the commercial field (here as union, not intersection), in programming languages as address operator or for linking strings. The "||" is used in (some) programming languages, sometimes also the "|". In my opinion, this mess only increases the confusion.

Why is it too much to ask to learn these two symbols (union, intersection) again, actually they should be known from school anyway (7th grade at grammar school in Germany). And there is a helpful tooltip.

[parafin]

People tend to forget things. The problem with existing symbols I see is that they are too similar. And not “obvious”.

Another idea - make symbols like Venn diagram (with filled either both circles or only their intersection).

[jenshannoschwalm]

I simply don't know.

A simple "&" is not good to me, more correct would be "&&" if we use "||" anyway.

Venn diagrams might be better than what we have now.

[pehar1]

Venn diagrams, in contrast to the ampersand, are logically correct representations. They therefore seem to me to be a good alternative to the currently used union and intersection symbols.

[ralfbrown]

We already have Venn-diagram icons in the mask manager which we can reuse if we go that way. The drawing code is in functions dtgtk_cairo_paint_masks_{union,intersection}.

[zisoft]

Next proposal:
Commit 8b72e9b uses Venn diagrams for union and intersection:

[TurboGit]

This sounds better to me. @ralfbrown @jenshannoschwalm @victoryforce what you view on this?

[victoryforce]

@TurboGit it's perfect for me.

[jenshannoschwalm]

Can't think of anything better now, let's take it.

[TurboGit]

Thanks!

[TurboGit]

@elstoc : I was about to add the documentation-pending tag that I had forgotten yesterday!

[zisoft]

Release notes:

Replaced the icon of the operator button in the color label filter for working with multiple selected color labels (union/intersection).

[TurboGit]

Thanks!