gtk: add symbolic icon

gtk: add symbolic icon

The -symbolic icon variant (if available) is used in the GNOME top bar,
and when the high contrast theme is in use.  This icon was created by
Jakub Steiner, and comes from the gnome-icons repository:

https://github.com/gnome-design-team/gnome-icons/blob/master/apps-symbolic/Adwaita/scalable/apps/transmission-symbolic.svg
https://bugzilla.redhat.com/show_bug.cgi?id=1221292

There is some confusion over whether symbolic app icons should be
installed to icons/hicolor/scalable/apps (alongside the regular scalable
icon) or to icons/hicolor/symbolic/apps. On the one hand,
https://wiki.gnome.org/Initiatives/GnomeGoals/HighContrastAppIcons has
this to say:

> […] obtain a suitable symbolic style icon […] and install it to the
> hicolor prefix, the same way you would for the full color variant.
>
> cp myapp-symbolic.svg /usr/share/icons/hicolor/scalable/apps/myapp-symbolic.svg

On the other hand, the Fedora package at
https://src.fedoraproject.org/rpms/transmission/blob/master/f/transmission.spec
ships this icon in icons/hicolor/symbolic/apps:

> # Install the symbolic icon
> mkdir -p  %{buildroot}%{_datadir}/icons/hicolor/symbolic/apps
> cp %{SOURCE1} %{buildroot}%{_datadir}/icons/hicolor/symbolic/apps/transmission-symbolic.svg

Anecdotally, icons in scalable/ have minimum size 64×64 on openSUSE, so
symbolic/ is the safer location (given the GNOME top bar uses 32×32
icons). This has the advantage of matching the location used in the
distribution which already ships this file.

https://github.com/transmission/transmission/issues/414