tweak media import dialog
tweak media import dialog

* make dialog resizable
* enable double-click on icons
* prevent error if no icon is selected
* prevent error if icon is dragged
* prevent error if icon is right-clicked / ctrl-clicked
* adjust background color
* adjust icon layout

ericrosenbaum/BeetleBlocks,ericrosenbaum/BeetleBlocks,jmoenig/Snap--Build-Your-Own-Blocks,jmoenig/Snap--Build-Your-Own-Blocks