Control glib symbol visibility

Hide symbols in versions of glib newer than (shortly after) the minimum
supported version, and don't warn if we use symbols deprecated in newer
glib.
Use Requests library for HTTP requests from Python
Add pylint suppressions for 0.26
Rename ErrorWindow to FatalErrorWindow
Add screenshot button to toolbar

Closes #11.
Store action group on VMWindow
Dim screenshot and restart buttons when VNC is disconnected