Move InstallerInterface to common

[danirabbit]

Throws a bunch of cool C errors:

DistinstBackend.c: In function ‘_dbus_installer_daemon_distinst_backend_get_disks’:
DistinstBackend.c:6249:107: error: ‘_dbus_installer_daemon_distinst_backend_get_disks_ready’ undeclared (first use in this function); did you mean ‘_dbus_installer_daemon_distinst_backend_get_disks’?
DistinstBackend.c:6249:107: note: each undeclared identifier is reported only once for each function it appears in
DistinstBackend.c: In function ‘_dbus_installer_daemon_distinst_backend_decrypt_partition’:
DistinstBackend.c:6388:145: error: ‘_dbus_installer_daemon_distinst_backend_decrypt_partition_ready’ undeclared (first use in this function); did you mean ‘_dbus_installer_daemon_distinst_backend_decrypt_partition’?
DistinstBackend.c: In function ‘_dbus_installer_daemon_distinst_backend_get_logical_device’:
DistinstBackend.c:6441:104: error: ‘_dbus_installer_daemon_distinst_backend_get_logical_device_ready’ undeclared (first use in this function); did you mean ‘_dbus_installer_daemon_distinst_backend_get_logical_device’?
DistinstBackend.c: In function ‘_dbus_installer_daemon_distinst_backend_install_with_default_disk_layout’:
DistinstBackend.c:6562:198: error: ‘_dbus_installer_daemon_distinst_backend_install_with_default_disk_layout_ready’ undeclared (first use in this function); did you mean ‘_dbus_installer_daemon_distinst_backend_install_with_default_disk_layout’?
DistinstBackend.c: In function ‘_dbus_installer_daemon_distinst_backend_install_with_custom_disk_layout’:
DistinstBackend.c:6734:228: error: ‘_dbus_installer_daemon_distinst_backend_install_with_custom_disk_layout_ready’ undeclared (first use in this function); did you mean ‘_dbus_installer_daemon_distinst_backend_install_with_custom_disk_layout’?
DistinstBackend.c: In function ‘_dbus_installer_daemon_distinst_backend_set_demo_mode_locale’:
DistinstBackend.c:6798:110: error: ‘_dbus_installer_daemon_distinst_backend_set_demo_mode_locale_ready’ undeclared (first use in this function); did you mean ‘_dbus_installer_daemon_distinst_backend_set_demo_mode_locale’?
DistinstBackend.c: In function ‘_dbus_installer_daemon_distinst_backend_trigger_demo_mode’:
DistinstBackend.c:6843:86: error: ‘_dbus_installer_daemon_distinst_backend_trigger_demo_mode_ready’ undeclared (first use in this function); did you mean ‘_dbus_installer_daemon_distinst_backend_trigger_demo_mode’?