Ability to use custom leaf object

[friggog]

Also from #48

Currently we get the shape here:

tree-gen/parametric/gen.py

Lines 293 to 298 in 0b0bc62

	# go through global leaf array populated in branch making phase and add polygons to mesh
	base_leaf_shape = Leaf.get_shape(self.param.leaf_shape, self.tree_scale / self.param.g_scale,
	self.param.leaf_scale, self.param.leaf_scale_x)
	base_blossom_shape = Leaf.get_shape(-self.param.blossom_shape, self.tree_scale / self.param.g_scale,
	self.param.blossom_scale, 1)

We should be able to alter the UI to let the user select a mesh object from the scene then get the mesh vertices to use as the base shape instead of getting the mesh from the cached list as currently happens.