Removing internal and private symbols from geary-static.vapi results
in removing 1,000 lines from the file.  Although testing doesn't show
an appreciable increase in compile time (w/ make -j8 on my 8-core
machine), by reducing symbol leakage partial builds should improve.

This patch also fixes a couple of symbols marked internal but being
used publicly (and necessarily so) and a couple of errordomains in
the Engine not placed in the Geary namespace.