caldav: extend query filter types

The basic types related to queries and filtering are missing some
features specified in the RFC (as also noted in the TODO comments). This
adds several of the missing elements, working towards being able to
handle all RFC-compliant queries.

The work is not fully done, e.g. the collation for text-match is still
not handled, but it's getting pretty close.

caldav/caldav.go

@@ -27,19 +27,30 @@ type CalendarCompRequest struct {
 }
 
 type CompFilter struct {
-	Name       string
-	Start, End time.Time
-	Props      []PropFilter
-	Comps      []CompFilter
+	Name         string
+	IsNotDefined bool
+	Start, End   time.Time
+	Props        []PropFilter
+	Comps        []CompFilter
+}
+
+type ParamFilter struct {
+	Name         string
+	IsNotDefined bool
+	TextMatch    *TextMatch
 }
 
 type PropFilter struct {
-	Name      string
-	TextMatch *TextMatch
+	Name         string
+	IsNotDefined bool
+	Start, End   time.Time
+	TextMatch    *TextMatch
+	ParamFilter  []ParamFilter
 }
 
 type TextMatch struct {
-	Text string
+	Text            string
+	NegateCondition bool
 }
 
 type CalendarQuery struct {