Fix README heading markup

Update source repo checkout command

Fix GTK2 icon browser crash.

Fix --list prints empty cell as "(null)"

To illustrate the issue before and after this fix:

    # /usr/bin/yad --list --print-all --column=A a '' c
    a|
    (null)|
    c|
    # ./src/yad --list --print-all --column=A a '' c
    a|
    |
    c|

The empty string '' in the second row results in a NULL C pointer,
which g_printf prints as "(null)". The fix consists of passing the
empty string "" to g_printf when the pointer is NULL.

For the GTK2 icon browser only, selecting an icon crashed yad.

The repo on GitHub seems canonical now, use this URL instead of the superseded Sourceforge one.
Signed-off-by: 林博仁(Buo-ren, Lin) <Buo.Ren.Lin@gmail.com>

Signed-off-by: 林博仁(Buo-ren, Lin) <Buo.Ren.Lin@gmail.com>

Fix configure.ac no sourceview support.

Fix conditional label alignment for GTK2

Fix GTK2 warning message