glib-demo: Make the Remove annotation button prominent in demo

Move out from Annotation properties to the top. This make better
use of the space.
Make the remove button active only if there is an annotation
selected.

https://bugs.freedesktop.org/show_bug.cgi?id=69978
glib-demo: Fix trailing whitespaces in main.c

https://bugs.freedesktop.org/show_bug.cgi?id=71245
glib-demo: Fix a typo in function name

https://bugs.freedesktop.org/show_bug.cgi?id=71245
glib-demo: Fix conding style issue in main.c

https://bugs.freedesktop.org/show_bug.cgi?id=71245