Mesh convert plugin

Implemented MeshConversion plugin which converts PolyMesh to TriMesh and vice versa.

The conversion function is provided via the scriptinterface and accepts an objectid as well as a bool to indicate whether the object should be converted to triMesh or to PolyMesh. The return value is either the id of the converted mesh, or -1 in case of error.

Also adds buttons to the toolbar for quick conversion of meshes selected as target (also works with multiple targets).

Note: The triangulation of polymeshes is done using the triangulate function in OpenMesh, hence only convex faces are triangulated properly for now.