improve parsing of desktop entries.

- speed up parsing by not creating nearly as many strings.
- fix basic whitespace handling. fixes #4

goos: linux
goarch: amd64
pkg: github.com/nwg-piotr/nwg-drawer
cpu: AMD Ryzen 7 1800X Eight-Core Processor
BenchmarkDesktopEntryParserOld-16    	   10000	    146094 ns/op
BenchmarkDesktopEntryParser-16       	   26097	     43303 ns/op
PASS
ok  	github.com/nwg-piotr/nwg-drawer	3.090s

main.go

@@ -59,6 +59,7 @@ type desktopEntry struct {
 	CommentLoc string
 	Icon       string
 	Exec       string
+	Category   string
 	Terminal   bool
 	NoDisplay  bool
 }