Implemented support for saving and loading drawn items

Drawn items can be saved to a file and these files can be loaded again. This could possibly improve the process of planning larger actions as plan can be stored for later use or shared with collaborators.

Downloading does not yet work properly on android, but as the whole draw-tools plugin is quite buggy on up-to-date phones (incorrect layout on phones/tablets with full hd resolution [at least on my htc one and nexus 7 fhd], drawing does not seem to work at all on kitkat devices as click events seem to be ignored) i hope this won't be much of a problem. Maybe i'll have time to take a look at all these general android problems with the draw-tools plugin and implement proper downloading support on android.