Make project import interactive

[tsmaeder]

Right now, importing Java projects is happening at startup. I am aware that there is the "interactive" mode, but all that allows me to do is to veto the import of the projects. (If I select "later", how do I later import the projects? There seems to be no "Import Projects" command 🤷 )

Let's consider making the import more interactive by

• allow the user to pick which directories will be imported
• allow the user to select which import will import a directory is two or more want to import a given directory
• allow importers to ask the user to fix problems while importing (think "no compatible Java version" could be fixed by selecting a JDK directory).

In order to implement something like this, we'd have to move control of the import process to the vscode-java side instead of handling import in the language server. That would also alllow us to give more precise feedback if something goes wrong with the import. IMO.

[snjeza]

allow the user to pick which directories will be imported
allow the user to select which import will import a directory is two or more want to import a given directory

Related to #3356

[tsmaeder]

@snjeza while these settings are useful, they don't change the fact that from the perspective of vscode-java, the import process is entirely opaque (except progress messages, which carry no semantics, as far as the IDE is concerned).