moved icons outside of source folder as they are not part of the source
code
added grid icon
(taken from
http://publib.boulder.ibm.com/infocenter/dmndhelp/v6rxmx/index.jsp?topic=/com.ibm.btools.help.modeler.doc/doc/reference/editors/structureeditorbuttonsandicons.html)
fixed plugin id
added grid icon
Thanks to Yusuke Kamiyamane who designed the icon.
The license is Creative Commons (Attribution 3.0 Unported).

regarding issue #23
added snap to grid icon
Thanks to Yusuke Kamiyamane who designed the icon.
The license is Creative Commons (Attribution 3.0 Unported).

regarding issue #23
removed icon due to unknown license
Use icons for toggle buttons to reduce size.
fixes issue #23