libsysprof-gtk: start on GTK widgetry library

This is ultimately to replace libsysprof-ui, and will focus on the display
of capture files. Recording will be left to the sysprof application in
terms of UI, and other applications like IDEs will use the
libsysprof-profile API directly along with libsysprof-gtk for visualizing.

src/meson.build

@@ -75,6 +75,7 @@ if get_option('libsysprof') or get_option('agent')
 endif
 if get_option('gtk') and get_option('libsysprof')
   subdir('libsysprof-ui')
+  subdir('libsysprof-gtk')
 endif
 if get_option('gtk') and get_option('libsysprof')
   subdir('sysprof')