Add .gitignore file for most common patterns/generated files.

.gitignore

+Makefile
+Makefile.in
+aclocal.m4
+autom4te.cache
+compile
+config.guess
+config.h
+config.h.in
+config.log
+config.status
+config.sub
+configure
+configure.in
+depcomp
+install-sh
+intltool-*
+libtool
+ltmain.sh
+mkinstalldirs
+missing
+stamp-h1
+.*.swp
+core.*
+core
+*.core
+*.service
+*.policy
+gtk-doc.make
+*-marshal.[ch]
+*_ui.h
+*-ui.h
+*-infos.h
+*.desktop
+*.desktop.in
+libxfce4panel*.pc
+*.o
+*.lo
+*.la
+.libs
+.deps
+INSTALL
+configure.ac
+gtk-doc.m4
+po/POTFILES
+po/stamp-it
+po/Makefile.in.in
+po/.intltool-merge-cache
+po/*.gmo
+docs/references/version.xml
+migrate/default.xml
+libxfce4panel/libxfce4panel-alias.h
+libxfce4panel/libxfce4panel-aliasdef.c
+libxfce4panel/libxfce4panel-enum-types.c
+libxfce4panel/libxfce4panel-enum-types.h
+libxfce4panel/libxfce4panel-config.h
+migrate/migrate
+panel/xfce4-panel
+plugins/applicationsmenu/xfce4-popup-applicationsmenu
+plugins/directorymenu/xfce4-popup-directorymenu
+plugins/windowmenu/xfce4-popup-windowmenu
+wrapper/wrapper-1.0