Add import to support freezing on Windows.
Windows compatibility hack for glReadPixels.
Refactor with tools and io as plugins.
Adds a "plugin loader" which actually just imports plugins.

We use this static import rather than dynamic loading because
cx-freeze seems to have problems with dynamic loading. Will solve
this later.
Don't create unneeded instances of the plugin api
Prompt before application exit if there is unsaved data.
Adds get/set config settings to the plugin api.
Rename "load_tool" to register_tool, minor tidy.
Adds flood fill tool.
Adds colour picker tool.
Can select background colour for display, closes #8.
Adds UI elements for setting grid size, see #12
Can specify a model size, closes #12
Adds "get bounding box" to model resize dialog.
Implements voxel space resizing.
Adds OBJ mesh export, still need colours adding, see #13
Exports material file along with OBJ export, closes #13
Fixes to freeze script to allow windows binaries.
Adds ambient occlusion render effect, closes #10
Fix random changing of the grid colour.