Qt: Add stock icons.

The new icons are patterned after the existing capture icons. They could
probably benefit from some adjustments here and there.

Start moving toward icon names and a directory layout compliant with
freedesktop.org's Icon Naming Specification and Icon Theme
Specification. We aren't fully compliant and might never be but anyone
with exposure to FDO icon themes should at least know where everything
is.

Make Capture Start (x-capture-start) the first icon in the toolbar.

Define the Colorize Packets, Auto Scroll, and zoom icons even though the
Qt UI doesn't use them yet.

Leave the Capture Filter, Display Filter, Coloring Rules, Preferences,
and Help icons off for now. The GTK+ toolbar is overly cluttered and I'm
not sure they're necessary.

Try not to break ui/gtk/toolbar_icons.h.

Remove welcome.qrc. I initially added it in case we needed to overlay
the welcome screen with translucent .pngs but that never happened.

To do:

- Move the old GTK+ icons to their own directory.

- Find a better name for the "toolbar" directory. "stock"?

- Make the toolbar configurable.

Change-Id: Ie07592113d307b8db786aedace672312a870fe38
Reviewed-on: https://code.wireshark.org/review/4182
Reviewed-by: Gerald Combs <gerald@wireshark.org>