ENH: Add search to extension manager

Note that this requires that the extension server also support search
functionality.

Issue #3564

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23128 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Fix missing search icon on some platforms

Use the CTK built-in fallback icon for the clear button in case loading
of the themed icon fails. This avoids having an invisible active click
area.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23129 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Disable search on extension details page

Tweak handling of the extension search field to disable it when looking
at the detail page of an extension, as it makes no sense in this case.
Also disable it (for now) when managing installed extensions.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23130 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Don't rely on URL change to update search

Also set the current search text (used to avoid unnecessary updates)
when applying the search filter. This way, the test still works if we
never get the URL changed signal (which seems to happen on some machines
for reasons unknown). In theory, since the URL change is being triggered
by JavaScript and not waiting on a server response, it should happen
immediately anyway, possibly even synchronous in the call to
evaluateJavaScript().

This should fix inability to clear the search text on some machines
(because failure to update the last search text means we always think
we have no search, and therefore don't need to do anything to clear the
search text).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23131 3bd1e089-480b-0410-8dfb-8563597acbee