Include only the required GTK icons in the Windows and Mac versions

gPodder for Windows currently ships with the whole Adwaita icon set (thousands of files). If possible, this set should be trimmed to only the icons that the application actually uses to make installing/unzipping/updating/backing up/moving gPodder to another folder much faster.

That is easier said than done. There is no way to tell what icons gpodder uses. Someone could visually look at the UI and make a list but then we would need to update that list everytime icons are changed, added or removed. No one will remember to do that and the Windows and Mac builds would be missing icons.

After the release, I'd like to change all icons to their "symbolic" variant and that would make it possible to automate the process by scanning all files for "*-symbolic".

I don't know if Windows and Mac users can install third-party extensions, but this would cause missing icons if the extensions use any not installed by gpodder.

There is no way to tell what icons gpodder uses.

This can actually be done by just monitoring the process:

However, if you can automate it, that's a better idea.

On gPodder 3.11.0 on Windows, when enabling the toolbar (Ctrl+T), the preferences icon is missing:

This seems to be preferences-desktop:

<object class="GtkToolButton" id="toolPreferences">
  <property name="visible">True</property>
  <property name="can-focus">False</property>
  <property name="action-name">app.preferences</property>
  <property name="label" translatable="yes">Preferences</property>
  <property name="icon-name">preferences-desktop</property>
</object>

That icon doesn't exist for 64x64 and larger icon sizes, and it appears gtk doesn't fallback to smaller icons. Maybe we should switch to preferences-other-symbolic.

#1404 makes this switch and changes the other toolbar icons to symbolic to match.