draw the crosshair toolbar icon from inline XPM data

The crosshair toggle button loaded the "crosshairs" icon from the
icon theme inside a bare try/except, so on systems without that
icon the button was empty and invisible. Replace it with a
self-contained red reticle (circle, center dot, cross arms) drawn
from inline XPM data, and set always-show-image so the desktop
default gtk-button-images=False cannot hide it.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/damaris/gui/DamarisGUI.py

@@ -17,6 +17,7 @@ import tempfile
 import gi
 gi.require_version('Gtk', '3.0')
 gi.require_version('GtkSource', '3.0')
+gi.require_version('GdkPixbuf', '2.0')
 
 # import 3rd party modules
 # gui graphics
@@ -32,6 +33,7 @@ if gtk_version_missmatch:
     raise Exception( "insufficient gtk version: " + gtk_version_missmatch )
 
 from gi.repository import Gdk as gdk
+from gi.repository import GdkPixbuf
 
 from gi.repository import GtkSource as gtksourceview
 
@@ -2976,6 +2978,39 @@ pygobject version %(pygobject)s
         configfile.write( "</damaris>\n" )
 
 
+# crosshair icon for the display toolbar, drawn from inline XPM data
+# so it does not depend on an installed icon theme
+crosshair_xpm = (
+    "24 24 2 1",
+    "  c None",
+    ". c #cc0000",
+    "           ..           ",
+    "           ..           ",
+    "           ..           ",
+    "           ..           ",
+    "           ..           ",
+    "                        ",
+    "                        ",
+    "         ......         ",
+    "        .      .        ",
+    "       .        .       ",
+    "       .  ....  .       ",
+    ".....  .  ....  .  .....",
+    ".....  .  ....  .  .....",
+    "       .  ....  .       ",
+    "       .        .       ",
+    "        .      .        ",
+    "         ......         ",
+    "                        ",
+    "                        ",
+    "           ..           ",
+    "           ..           ",
+    "           ..           ",
+    "           ..           ",
+    "           ..           ",
+    )
+
+
 class MonitorWidgets:
     """Manages the matplotlib-based data visualization panel.
 
@@ -3082,13 +3117,10 @@ class MonitorWidgets:
         self.matplot_toolbar.remove(self.matplot_toolbar.get_nth_item(6))
         # now let's add a button to the toolbar
         self.matplot_cursor_button = gtk.ToggleButton(label='')
-        try:
-            icon_theme = gtk.IconTheme.get_default()
-            icon = icon_theme.load_icon("crosshairs", 24, 0)
-            image = gtk.Image.new_from_pixbuf(icon)
-            self.matplot_cursor_button.set_image(image)
-        except:
-            pass
+        cursor_icon = GdkPixbuf.Pixbuf.new_from_xpm_data( crosshair_xpm )
+        self.matplot_cursor_button.set_image( gtk.Image.new_from_pixbuf( cursor_icon ) )
+        # gtk-button-images is False on many desktops, which would hide the icon
+        self.matplot_cursor_button.set_always_show_image( True )
         self.matplot_cursor_button.set_tooltip_text('Enable Crosshairs')
         self.matplot_cursor_button.show()
         self.matplot_cursor_button.connect('toggled', self.toggle_cursor)