Remove duplicate and conflicting definition of ResizeHandle

There were two definitions of ResizeHandle, one in a separate file and
one in windowmanager.js. Before django-pipeline was used, the latter one
was used. However, after django-pipeline was included it was not
deterministic anymore which definition took precedence. That is because
all files get concatenated into one. Apparently the compiler choose
sometimes (usually) the wrong one. This commit fixes this by moving the
definition from within windowmanager.js into the separate file,
effectively overriding the second definition and removing the first one.
Move resize handler back into window manager

The window manager is too a large part self-contained. Having the resize handle
in a different file breaks this and since the resize handle is only used by the
window manager it fits better in its file than in its own.
Merge remote-tracking branch 'origin/master' into one-micron