[feature] Add export dialog with options

[skylot]

It will be nice to have a single export dialog with options:

• export as a Gradle project
• don't save classes from excluded packages
• don't save resources
• maybe others

Now excluded packages not applied, and other options only available in preferences dialog.
Also, current name Save All not clear and can interfere with saving project actions, so it is better to rename it to Export.

Original discussion started in #1085 (comment)