More flexible dark scheme handling

It works now as follows:
1) try to get theme information from the system, and if present us it.
2) if no theme information of the system is present, take the one from
   the musredit_startup.xml.

Now it is possible to switch the icon theme for the menu and toolbar
separately.

src/musredit_qt5/musredit/musredit_startup.xml.in

@@ -17,7 +17,8 @@
     <musrview_show_fourier>n</musrview_show_fourier>
     <musrview_show_avg>n</musrview_show_avg>
     <enable_musrt0>y</enable_musrt0>
-    <dark_theme_icons>n</dark_theme_icons>
+    <dark_theme_icons_menu>n</dark_theme_icons_menu>
+    <dark_theme_icons_toolbar>n</dark_theme_icons_toolbar>
   </general>
   <recent_files>
     <path_file_name>@DOCDIR@/examples/test-histo-PSI-BIN.msr</path_file_name>