feat(plugins): Add initial version of ViewportPlugin class (#242)

* Add initial version of ViewportPlugin class
* Move entrypoints from OHIFPlugin to OHIF.plugins
* Add URL route to DICOMWeb server for Standalone Viewer

Packages/ohif-viewerbase/client/lib/classes/plugins/OHIFPlugin.js

@@ -0,0 +1,48 @@
+export class OHIFPlugin {
+    // TODO: this class is still under development and will
+    // likely change in the near future
+    constructor () {
+        this.name = "Unnamed plugin";
+        this.description = "No description available";
+    }
+
+    // load an individual script URL
+    static loadScript(scriptURL) {
+        const script = document.createElement("script");
+
+        script.src = scriptURL;
+        script.type = "text/javascript";
+        script.async = false;
+
+        const head = document.getElementsByTagName("head")[0];
+        head.appendChild(script);
+        head.removeChild(script);
+
+        return script;
+    }
+
+    // reload all the dependency scripts and also
+    // the main plugin script url.
+    static reloadPlugin(plugin) {
+        console.warn(`reloadPlugin: ${plugin.name}`);
+        if (plugin.scriptURLs && plugin.scriptURLs.length) {
+            plugin.scriptURLs.forEach(scriptURL => {
+                this.loadScript(scriptURL).onload = function() {}
+            });
+        }
+
+        let scriptURL = plugin.url;
+
+        if (plugin.allowCaching === false) {
+            scriptURL += "?" + performance.now();
+        }
+
+        this.loadScript(scriptURL).onload = function() {
+            const entryPointFunction = OHIF.plugins.entryPoints[plugin.name];
+
+            if (entryPointFunction) {
+                entryPointFunction();
+            }
+        }
+    }
+}