Install XDG desktop icon files.

The icon files will be installed when running make install or cabal
install. Did not try to run update-icon-caches, since I think it's debian
specific, and dh_icons will take care of that for the Debian package.

Using the favicon as a 16x16 icon. At 24x24 the svg displays pretty well,
although the dotted lines are rather faint. The svg is ok at all higher
resolutions.

The standalone linux build auto-installs the desktop and autostart files
when run. I have not made it auto-install the icon file too, because
a) that would take more work to include them in the tarball and find them
b) it would need to be an install to ~/.icons/, and I don't know if that
   really works!

Utility/FreeDesktop.hs

@@ -3,6 +3,7 @@
  - http://standards.freedesktop.org/basedir-spec/latest/
  - http://standards.freedesktop.org/desktop-entry-spec/latest/
  - http://standards.freedesktop.org/menu-spec/latest/
+ - http://standards.freedesktop.org/icon-theme-spec/latest/
  -
  - Copyright 2012 Joey Hess <joey@kitenet.net>
  -
@@ -16,6 +17,7 @@ module Utility.FreeDesktop (
 	writeDesktopMenuFile,
 	desktopMenuFilePath,
 	autoStartPath,
+	iconFilePath,
 	systemDataDir,
 	systemConfigDir,
 	userDataDir,
@@ -34,6 +36,7 @@ import System.Directory
 import System.FilePath
 import Data.List
 import Data.String.Utils
+import Data.Maybe
 import Control.Applicative
 
 type DesktopEntry = [(Key, Value)]
@@ -54,18 +57,19 @@ toString (ListV l)
   where
 	escapesemi = join "\\;" . split ";"
 
-genDesktopEntry :: String -> String -> Bool -> FilePath -> [String] -> DesktopEntry
-genDesktopEntry name comment terminal program categories =
+genDesktopEntry :: String -> String -> Bool -> FilePath -> Maybe String -> [String] -> DesktopEntry
+genDesktopEntry name comment terminal program icon categories = catMaybes
 	[ item "Type" StringV "Application"
 	, item "Version" NumericV 1.0
 	, item "Name" StringV name
 	, item "Comment" StringV comment
 	, item "Terminal" BoolV terminal
 	, item "Exec" StringV program
+	, maybe Nothing (item "Icon" StringV) icon
 	, item "Categories" ListV (map StringV categories)
 	]
   where
-	item x c y = (x, c y)
+	item x c y = Just (x, c y)
 
 buildDesktopMenuFile :: DesktopEntry -> String
 buildDesktopMenuFile d = unlines ("[Desktop Entry]" : map keyvalue d) ++ "\n"
@@ -89,6 +93,17 @@ autoStartPath :: String -> FilePath -> FilePath
 autoStartPath basename configdir =
 	configdir </> "autostart" </> desktopfile basename
 
+{- Path to use for an icon file.
+ - The resolution is something like "48x48" or "scalable". -}
+iconFilePath :: FilePath -> String -> Bool -> IO FilePath
+iconFilePath file resolution systemwide
+	| systemwide = return $ systemDataDir </> "icons" </> subpath
+	| otherwise = do
+		home <- myHomeDir
+		return $ home </> ".icons" </> subpath
+  where
+	subpath = "hicolor" </> resolution </> "apps" </> file
+
 desktopfile :: FilePath -> FilePath
 desktopfile f = f ++ ".desktop"