Improve props handling for <once-button> component

now we use computed properties for some of the underlying button props.  this
also adds a "click" event for the element; callers should be able to add
handler for that which happens *in addition to* the button disabling.  it's
assumed that's always safe or else caller wouldn't use `<once-button>`

tailbone/static/js/tailbone.buefy.oncebutton.js

@@ -8,10 +8,10 @@ const OnceButton = {
         ':tag="tag"',
         ':href="href"',
         ':title="title"',
-        ':disabled="disabled"',
+        ':disabled="buttonDisabled"',
         '@click="clicked"',
         '>',
-        '{{ text }}',
+        '{{ buttonText }}',
         '</b-button>'
     ].join(' '),
 
@@ -27,17 +27,39 @@ const OnceButton = {
         disabled: Boolean
     },
 
+    data() {
+        return {
+            currentText: null,
+            currentDisabled: null,
+        }
+    },
+
+    computed: {
+        buttonText: function() {
+            return this.currentText || this.text
+        },
+        buttonDisabled: function() {
+            if (this.currentDisabled !== null) {
+                return this.currentDisabled
+            }
+            return this.disabled
+        },
+    },
+
     methods: {
 
         clicked(event) {
-            this.disabled = true
+            this.currentDisabled = true
             if (this.workingText) {
-                this.text = this.workingText
+                this.currentText = this.workingText
             } else if (this.working) {
-                this.text = this.working + ", please wait..."
+                this.currentText = this.working + ", please wait..."
             } else {
-                this.text = "Working, please wait..."
+                this.currentText = "Working, please wait..."
             }
+            this.$nextTick(function() {
+                this.$emit('click', event)
+            })
         }
     }