Project UI improvement

[ActiveChooN]

My actions before raising this issue

• Read/searched the docs
• Searched past issues

Some @zhiltsov-max's comments about improving project export UI:

1. How about renaming "export task dataset" to "export (as) dataset"? Two very similar names confuse.
   
   For example it's good idea to rename as @nmanovic suggested in Project export #3365 (comment)

2. After the button is pressed and the menu disappears, when I reopen the menu, it has the button highlighted.
   

3. The display of subset names looks a bit unclear to me. Maybe, it should be improved.
   

4. I didn't see 2d / 3d task / project indicator anywhere. Maybe, it's worth adding.

5. Rename tasks default subsets to train, test and val to match format default names.

Originally posted by @zhiltsov-max in #3502 (comment)