Add ability to keep all opened files after project's close.

After update 4.9. beta, I noticed change introduced in QTCREATORBUG-15721, which not suitable for me:

When you close project, ALL your open files are closed too! I don't want to close all my files.

My common scenario:

• open project;
• open some files , work-work;
• create some commits an a branch;
• after work is done, I need to rebase my branch. So I close Qt Creator
  project, do rebase and open it again. I usually still have all my files
  opened!

I open project to use some files from it. I don't want to appear files in locator, if i working on 5 projects, all files will be in a search, which is annoying.

Also, code mode is overwhelming with huge files count, it just works faster when only one project is opened (and you don't see usages from closed project when using "find references" on common header file).

So, I open project, find file in locator, add to pend files, then just want to close project and use file in split view to compare some code.

I almost always have some non-project files kept open, so new "feature" is really annoying.

Please consider creating option to disable current behavior.