Rename desktop file to GApplication application ID

This is needed in order to advertise that we support DBus
activation.
Make Boxes DBus activatable
app: Setup brokers during activation

Setup of brokers imply creation of Machine instances and they assume
App.main_window to be non-null but App.main_window is only initialized
for the first time during 'activate' so setup of brokers can easily
crash Boxes if done through 'startup' (which happens before 'activate').