Add Dashboard Icons prefix support

- defined new type IconSource and constants, presently supporting:
  - LocalFile
  - SimpleIcon
  - DashboardIcon
- added new field to bookmarks and monitors to hold IconSource
- adjusted IsSimpleIcon to get truthiness from IconSource field
- generalised `toSimpleIconIfPrefixed` into `toRemoteResourceIconIfPrefixed`,
  adding support for `walkxcode`'s dashboard icons via CDN (svg)

internal/widget/monitor.go

@@ -49,6 +49,7 @@ type Monitor struct {
 		Title                   string           `yaml:"title"`
 		IconUrl                 string           `yaml:"icon"`
 		IsSimpleIcon            bool             `yaml:"-"`
+		IconSource              IconSource       `yaml:"-"`
 		SameTab                 bool             `yaml:"same-tab"`
 		StatusText              string           `yaml:"-"`
 		StatusStyle             string           `yaml:"-"`
@@ -61,7 +62,8 @@ func (widget *Monitor) Initialize() error {
 	widget.withTitle("Monitor").withCacheDuration(5 * time.Minute)
 
 	for i := range widget.Sites {
-		widget.Sites[i].IconUrl, widget.Sites[i].IsSimpleIcon = toSimpleIconIfPrefixed(widget.Sites[i].IconUrl)
+		widget.Sites[i].IconUrl, widget.Sites[i].IconSource = toRemoteResourceIconIfPrefixed(widget.Sites[i].IconUrl)
+		widget.Sites[i].IsSimpleIcon = widget.Sites[i].IconSource == SimpleIcon
 	}
 
 	return nil