Add few desktop files, this fixes bug:639 #9

I came up to this in order to fix bug639.

Everyone will get the same desktop files as well as the same icons.
The desktop files have been taken from your Debian packaging system.

Not sure It will work for the *BSD since they put their files in /usr/local thought.

Tell me what you think about this.

I've added a sharedir option to configure.

Something to test like:

./configure \
     -prefix "/usr" \
     -sysshare "/usr/share"

There are only small comments. Please consider naming the variable "sysshare" instead of "share" because its purpose is similar to "sysconf" variable.

@cethyel, please can you simplify the commit log a bit?

For example, it doesn't make much sense to include in one comment "add a sharedir option…" and then "rename share option to sysshare…", 3× the same "Signed-off-by", etc.

There is a small typo that requires correction.

Tested along with a patch for tde-packaging, there were no problems.

     sysconfdir)
 	QT_INSTALL_SYSCONF="$VAL"
 	;;
 	sharedir)

 # default PREFIX/etc/settings
 [ -z "$QT_INSTALL_SYSCONF" ]      && QT_INSTALL_SYSCONF=$QT_INSTALL_PREFIX/etc/settings
 # default PREFIX/usr/share
 [ -z "QT_INSTALL_SHARE" ]         && QT_INSTALL_SHARE=$$QT_INSTALL_PREFIX/usr/share

 # default PREFIX/etc/settings
 [ -z "$QT_INSTALL_SYSCONF" ]      && QT_INSTALL_SYSCONF=$QT_INSTALL_PREFIX/etc/settings
 # default PREFIX/share
 [ -z "QT_INSTALL_SHARE" ]         && QT_INSTALL_SHARE=$QT_INSTALL_PREFIX/share