Let "dub run" build a project in its own directory if possible

[s-ludwig]

Currently running "dub" or "dub run" will build a project in a temporary folder instead of the project folder as "dub build" does. The reason for this is to support running projects in directories where the user doesn't have write privileges and to avoid cluttering projects when they are just being run.

However, this has some drawbacks:

• Windows will create a new firewall entry per "dub run" because the executable lives in a different path each time
• Builds cannot be reused (currently only possible using "dub build --rdmd"

So even if the current behavior avoids cluttering the project folder by default, changing the behavior seems to be the better choice overall. If the project folder is not writable, the current behavior will stay as a fallback.