Add the ability to select alt-text

**Is your feature request related to a problem? Please describe.**
When I share an image post from Voyager to Tusky (Mastodon client), the alt-text doesn't come along.

**Describe the solution you'd like**
Improving the "share" feature would solve this I suppose, but the ability to select the alt-text (and thus to copy it) would also solve the problem and sounds useful on its own as well.

**Describe alternatives you've considered**
I haven't found an alternative at all so far. Even opening the page in Firefox mobile doesn't let me select (or even show!) the alt-text.

**Additional context**
Here is a typical image post with a long alt-text I really don't want to have to type manually :
https://jlai.lu/post/22231711