Ignore Visual Studio project files

With `make vs2013-ide-integration` dozens of VS specific
project and solution files are generated. These add noise
and time to `git status` and make it harder to locate
new files.

Restored the exclusion of .vcxproj and .sln files and
added windows/ directory to the ignore list.

Change-Id: I64abe395824503a5eda25d70d7467ba3fa4e4aa8
Reviewed-on: https://gerrit.libreoffice.org/19972Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>

.gitignore

@@ -116,9 +116,13 @@ DerivedData
 # MSVS specific
 */Debug
 *.sdf
+*.sln
 *.v11*
 *.v12*
+*.vcxproj*
 *.opensdf
+!windows/*.sln
+!windows/*.vcxproj*
 
 # QtCreator specific
 *.pro