Improve loading for tags

When opening the tag select and type something the tags are loaded.

If you continue to type but the tags are not finally loaded yet (the first xhr request is still running)
another xhr request is triggered. This happens for every keystroke.

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>

core/src/systemtags/systemtagscollection.js

@@ -69,7 +69,7 @@
 			fetch: function(options) {
 				var self = this
 				options = options || {}
-				if (this.fetched || options.force) {
+				if (this.fetched || this.working || options.force) {
 				// directly call handler
 					if (options.success) {
 						options.success(this, null, options)
@@ -79,10 +79,13 @@
 					return Promise.resolve()
 				}
 
+				this.working = true
+
 				var success = options.success
 				options = _.extend({}, options)
 				options.success = function() {
 					self.fetched = true
+					self.working = false
 					if (success) {
 						return success.apply(this, arguments)
 					}