Add .gitignore

This removes a lot of the temporary or autogenerated files from
the git status output which may also help prevent these files
from accidently being added into patches.

.gitignore

+data/appdata/xfce4-power-manager.appdata.xml
+data/interfaces/acme_ui.h
+data/interfaces/xfpm-settings_ui.h
+libdbus/org.xfce.unique.h
+libdbus/xfpm-dbus-marshal.c
+libdbus/xfpm-dbus-marshal.h
+src/panel-plugins/brightness/xfce4-brightness-plugin
+po/Makefile.in.in
+po/xfce4-power-manager.pot
+settings/xfce-power-manager-dbus-client.h
+settings/xfce4-power-manager-settings
+src/org.freedesktop.PowerManagement.Inhibit.h
+src/org.freedesktop.PowerManagement.h
+src/org.xfce.power.policy
+src/xfce-power-manager-dbus-client.h
+src/xfce-power-manager-dbus-server.h
+src/xfce4-power-information
+src/xfce4-power-manager
+src/xfpm-marshal.c
+src/xfpm-marshal.h
+src/xfpm-power-backlight-helper
+src/xfce4-pm-helper
+common/xfpm-enum-types.c
+common/xfpm-enum-types.h