Cache panel contents after first load

Previously:

1) Panels are by default conditionally rendered,
so each time a panel is closed and reopened,
its contents are recreated.

2) Else, panels can be preloaded if explicitly wanted.
However, this adds extra rendering time.

Now:

Case 2 remains the same.
Authors can still choose to preload their panels if they wish.

For Case 1, we cache the rendered content so as to prevent re-rendering,
which
  a) prevents multiple HTTP requests if using the src attribute
  b) solves issues that stem from the destruction and recreation of
     components:
     MarkBind/markbind#868
     MarkBind/markbind#483

Implementation:

Store an additional `isCached` property, which is set to true
whenever the content is fetched.

When `isCached` is true, we switch from
v-if (conditional rendering)
to
v-show (conditional display)
More info: https://vuejs.org/v2/guide/conditional.html#v-show

As a side effect, since once the panel is loaded v-show is always used,
the v-else part of the template is not needed anymore.

Future work:

Investigate the use of keep-alive:
https://vuejs.org/v2/guide/components-dynamic-async.html

Tried to surround the content with <keep-alive></keep-alive>
but did not seem to be cached.

src/Panel.vue

@@ -42,7 +42,7 @@
                     </slot>
                 </div>
             </div>
-            <template v-if="preloadBool">
+            <template v-if="preloadBool || isCached">
                 <div class="card-collapse"
                      ref="panel"
                      v-show="localExpanded"
@@ -57,21 +57,6 @@
                     <hr v-show="isSeamless" />
                 </div>
             </template>
-            <template v-else>
-                <div class="card-collapse"
-                     ref="panel"
-                     v-if="localExpanded"
-                >
-                    <div class="card-body">
-                        <slot></slot>
-                        <retriever v-if="hasSrc" ref="retriever" :src="src" :fragment="fragment" delay></retriever>
-                        <panel-switch v-show="isExpandableCard && bottomSwitchBool" :is-open="localExpanded"
-                                      @click.native.stop.prevent="collapseThenScrollIntoViewIfNeeded()"
-                                      @is-open-event="retrieveOnOpen"></panel-switch>
-                    </div>
-                    <hr v-show="isSeamless" />
-                </div>
-            </template>
         </div>
     </span>
 </template>
@@ -229,7 +214,8 @@
       return {
         onHeaderHover: false,
         localExpanded: false,
-        localMinimized: false
+        localMinimized: false,
+        isCached: false
       }
     },
     methods: {
@@ -270,6 +256,15 @@
     },
     watch: {
       'localExpanded': function (val, oldVal) {
+        if (this.isCached) {
+        	return;
+        }
+        /* When either one of the values is true,
+         * it means that the data is/will be cached.
+         */
+        if (val || oldVal) {
+          this.isCached = true;
+        }
         this.$nextTick(function () {
           this.retrieveOnOpen(this, val);
         })