Add an Inspector to Geary

This adds an Inspector window to Geary, activated using <Alt><Shift>I that displays runtime info (version numbers, desktop environment, etc) and gives access to Geary's debug logs, even if the app was not launched with `-d``, and allows filtering, copying and saving the logs & details to disk.

This ticks off a few things off of #354 (closed). It could be improved still, but is a good start.