Provide a flag to disable individual terms in a Glossary from being used as such

### Describe the problem

So well - I know: this sounds a bit nonsensical in the first moment. But let me explain:

We have a component "UI Essentials". It contains very elementary things like button captions (actions) and labels or state information. Obviously, these are exactly those kinds of terms for which you want to ensure that they are always translated in the same way => exactly one of the core purposes of a glossary.

Technically though, this is an actual string-set (coming straight from development) - it's not a TBX but in RESX format, as we are using it directly in our code. Gladly, Weblate is flexible enough, that we can still use this component as a glossary.

But here's the problem:

The stringset includes a number of terms which are too generic for a glossary (e.g. "New", "Add", "Check", etc), because they are appearing in everywhere in many other strings.
And these are very toxic for all projects using it as glossary: It is polluting and overflowing the translation editor (specifically the glossary sidebar) and it's masking/hiding all the "good" and useful glossary matches - not even talking about checks here...

It was unbearable for translators and so I had to disable it from being a glossary.

### Solution brainstorm

Essentially, it's just a small amount (maybe like 10%) of those terms which are responsible for the big mess.

But I can't just delete those 10% of entries - they are crucial and needed. Splitting is not an option either - neither feasible from the side of consuming code, nor from the Weblate side, because you don't know the offending terms in advance. And you can't move single terms from one stringset/component to another one on a weekly basis.

There's a simple remedy: all that is needed is a flag that allows to disable individual terms for glossary usage:

- They still appear in the glossary component and can be translated like normal
- But these disabled terms are excluded in all glossary use cases
- Not considered for checks
- Not shown in the glossary sidebar
- Not marked in the editor

### Describe alternatives you have considered

My other suggestion - [Provide a flag to control case sensitivity for individual Glossary terms](https://github.com/WeblateOrg/weblate/issues/16412) - can help with this a little bit, but it is focused on slightly different cases. It has its own justification, separate from this one.