Add gtkmm
The goal of this PR is to add gtkmm to the build snap. In order to accomplish this, the following parts have been added: mm-common, glibmm, cairomm, pangomm, atkmm, gtkmm.
This PR also includes some cleanup such as pinning each part to a git object (branch, tag, or hash), so we avoid any problems that would be caused by building from master (with the exception of libtool and libcanberra).