Cleanup: Detailed comments on selection helpers & simplify logic

Merge branch 'blender-v2.81-release'

Fix T70991: Maximized file browser hides file name bar on Windows

`WS_CHILD` is a different kind of child window that what we define as
child window. See http://forums.codeguru.com/showthread.php?491604.

Setting this style flag seems to mess things up a bit in our
configuration. The name bar is actually being overlapped by the Windows
task bar then. Not totally sure why this happens, but I think it's
because windows with the `WS_CHILD` style are positioned relative to the
parent, not the desktop (screen without taskbar). So it uses the full
space available when maximized, which isn't clipped by the taskbar
anymore.

Fix incorrect return value, cleanup return values

Update branch to include changes from split off patches

Fix T71590: Closing file browser reopens previously closed render view

The render view window was never closed actually, just moved behind the
main window. It's properly closed now.

It should also behave more like expected when there already is a
temporary window open (e.g. Preferences).

Fix incorrect use of use_handle option

Minor cleanup: Clarify logic

Cleanup: Clarify iterator visibility flag

Fix incorrect selection flag usage