some cleanup around changes
fixed bug with simpeleffects
implemented merging of layers. Closes #93
plugins can add custom importers/exporters, fixed bugs
added layer float/merge shortcuts and fixed MoveLayer
added preserving float and changed saveas shortcut to Ctrl+Alt+S
cleanup and add missing license headers
added clipboard support
disable loading of local/global plugins and fixed delete key function
update titlebar with document name
fixed layer bugs and adjusted floating layers
multi-document support!
clean up ImageImporter/Exporter
fixed history bug and change constants to non-debug values
added missing license headers
start with no image
more UI improvements
added tool settings
fix focus bugs
colour picker tweak