unix: add SetIconTheme and SetIconThemePath for SNI icon theme support

[vasyvasilie]

Problem

On MATE desktop (with mate-indicator-applet) and some GNOME setups, the
StatusNotifierItem spec allows displaying the icon via IconPixmap (raw ARGB
data) or via IconName + IconThemePath (XDG icon theme lookup). Many
indicator applets on these desktops prefer IconName and either ignore or
poorly render IconPixmap.

Currently systray hardcodes IconName: "" and IconThemePath: "" as
non-writable properties in createPropSpec(), with no public API to change
them. There is also no way to make IconPixmap return an empty array — it
always calls convertToPixels(t.iconData) even when you'd rather have the DE
(Desktop Environment) fall back to IconName.

Real-world impact

I built an ap using Fyne + systray. The app switches tray icons between
two states. On MATE with
mate-indicator-applet:

• IconPixmap is sent as a(iiay) with raw bytes, but the applet renders a
  blank/invisible icon
• verified via busctl get-property that IconName = "" and
  IconThemePath = "" at all times
• switching icons via systray.SetIcon(bytes) updates IconPixmap but the
  tray stays blank

The only working solution for MATE is:

# working state verified via busctl:
IconName:      "link"
IconThemePath: "/usr/local/bin/resources"
IconPixmap:    a(iiay) 0   ← empty, so DE uses IconName

Changes

• SetIconName(name string) — sets IconName DBus property and emits
  NewIcon signal to notify the host
• SetIconThemePath(path string) — sets IconThemePath DBus property
• iconPixmapValue(iconData []byte) []PX — returns empty []PX{} when no
  icon data (allows DE to fall back to IconName); returns normal pixmap
  otherwise
• createPropSpec: IconName and IconThemePath now initialized from
  package-level vars and marked Writable: true

Backward compatibility

Fully backward compatible. Default values of iconName and iconThemePath
are "" — existing behavior unchanged. iconPixmapValue only returns empty
array when iconData is nil/empty.

Environment

• OS: Ubuntu 24.04 LTS (Noble)
• Desktop: MATE 1.26.1 (mate-session-manager 1.26.1-2build2)
• Tray host: mate-indicator-applet 1.26.0-1build3 via Ayatana indicators
• Indicator stack:
  • libayatana-appindicator3-1 0.5.93-1build3
  • libayatana-indicator3-7 0.9.4-1build1
  • libdbusmenu-gtk3-4 18.10.20180917~bzr492+repack1-3.1ubuntu5
• SNI watcher: org.kde.StatusNotifierWatcher (ayatana-indicator process)

Diagnostics

Other apps on the same system (e.g. blueman-tray) correctly use IconName
without IconPixmap:

$ busctl --user get-property org.blueman.Tray /org/blueman/sni \
    org.kde.StatusNotifierItem IconName
s "blueman-active"

Our app before this patch (systray sets only IconPixmap, IconName = ""):

$ busctl --user get-property org.kde.StatusNotifierItem-{PID}-1 \
    /StatusNotifierItem org.kde.StatusNotifierItem IconName
s ""

$ busctl --user get-property org.kde.StatusNotifierItem-{PID}-1 \
    /StatusNotifierItem org.kde.StatusNotifierItem IconPixmap
a(iiay) 1 22 22 0 0 0 0 ...   ← present but ignored by MATE/Ayatana → blank icon

After this patch (SetIconName("link") + SetIconThemePath("/path/to/resources")):

$ busctl --user get-property org.kde.StatusNotifierItem-{PID}-1 \
    /StatusNotifierItem org.kde.StatusNotifierItem IconName
s "link"

$ busctl --user get-property org.kde.StatusNotifierItem-{PID}-1 \
    /StatusNotifierItem org.kde.StatusNotifierItem IconPixmap
a(iiay) 0   ← empty, Ayatana falls back to IconName → icon displayed correctly

Testing

Tested on MATE desktop with mate-indicator-applet (SNI via
org.kde.StatusNotifierItem). Icon switches correctly between states on
SetIconName calls, verified via busctl introspect.