BUG: Use a safe temporary path for download

Change extension downloading to use a temporary file for the download
location. This ensures that a: we don't clutter the temporary directory
with old downloads, b: multiple users can download the same extension
using a shared temporary directory (e.g. most UNIX-like systems), and c:
the file name is not predictable, which is a security vulnerability.

Issue #3552

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23041 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Install QWebView designer plugin to fix loading of UI file

By installing the designer plugin, we ensure that UI file having a
QWebView can successfully be loaded at runtime.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23042 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Fix install rule incorrectly set in r23042

Optional matching pattern are now specified after a semicolon.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23043 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Fix popups always on top

Closes #1792, #1824, #1763, #2077, #2309, #2474

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23044 3bd1e089-480b-0410-8dfb-8563597acbee