gtk3 sample: remove a trailing space
gtk3 sample: remove trailing blank lines
gtk3 sample: remove a trailing space
gtk3 sample: remove needless spaces

Now, our project uses the following style for a single line
block:

    signal_connect("...") {|x| ...}
    signal_connect("...") {...}
gtk3 sample: add missing spaces after comma
gtk3 sample: use spaces for indent
gtk3 sample: fix indent
gtk3 sample: fix if statement style
gtk3 sample: fold long lines
gtk3 sample: cleanup