feat: an app icon in colour and symbolic form, validated at build time

Both are drawn to the GNOME HIG: a 128px instrument face that leaves its
margins, carries one 4px front profile for depth and no gradients or shadows,
and a 16px symbolic reduction of the same metaphor in fills only. The colour
icon strokes its sine because it is never recoloured; the symbolic one cannot.

Every file in the manifest now preprocesses through xmllint, because a
malformed SVG is the one icon failure no runtime check can see: librsvg refuses
the file while the name still resolves, so the icon draws nothing and
has_icon() says it is fine.

CMakeLists.txt

@@ -39,7 +39,7 @@ if(CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")
   target_compile_options(udpsclient PRIVATE -Wall -Wextra)
 endif()
 
-# --- icons: four symbolic SVGs compiled into the binary -----------------------
+# --- icons: six SVGs compiled into the binary --------------------------------
 # There is no stock icon for an oscilloscope trigger, a pair of cursors or a measurement
 # ruler, so this app draws its own. They go in a GResource rather than beside the binary
 # because udpscope is run straight out of the build tree and has no install step: a
@@ -48,7 +48,8 @@ endif()
 find_program(GLIB_COMPILE_RESOURCES NAMES glib-compile-resources REQUIRED)
 set(UDPSCOPE_ICON_DIR ${CMAKE_CURRENT_SOURCE_DIR}/src/icons)
 file(GLOB UDPSCOPE_ICON_FILES CONFIGURE_DEPENDS
-     ${UDPSCOPE_ICON_DIR}/scalable/actions/*.svg)
+     ${UDPSCOPE_ICON_DIR}/scalable/actions/*.svg
+     ${UDPSCOPE_ICON_DIR}/scalable/apps/*.svg)
 add_custom_command(
   OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/udpscope_resources.c
   COMMAND ${GLIB_COMPILE_RESOURCES}