frontend: Provide UI for phase 1 of plugin manager

For phase 1 of the plugin manager, the ability to toggle off/on plugins
to be loaded at launch is provided.

This commit adds a new Plugin Manager dialog which can be accessed from
the Tools menu, which shows a list of all installed 3rd party plugins
with a checkbox to toggle them off or on.  If a change is made, the user
is prompted to restart OBS. To allow this, the plugin manager uses a
json based config file stored in the OBS config directory. Additionally
for sources in the source tree, a sample UI has been provided that
indicates any sources whose parent module is disabled, by turning its
title red.

frontend/components/SourceTreeItem.cpp

@@ -9,6 +9,8 @@
 #include <QLineEdit>
 #include <QPainter>
 
+#include "plugin-manager/PluginManager.hpp"
+
 #include "moc_SourceTreeItem.cpp"
 
 static inline OBSScene GetCurrentScene()
@@ -84,6 +86,16 @@ SourceTreeItem::SourceTreeItem(SourceTree *tree_, OBSSceneItem sceneitem_) : tre
 	label->setAttribute(Qt::WA_TranslucentBackground);
 	label->setEnabled(sourceVisible);
 
+	const char *sourceId = obs_source_get_unversioned_id(source);
+	switch (obs_source_load_state(sourceId)) {
+	case OBS_MODULE_DISABLED:
+	case OBS_MODULE_MISSING:
+		label->setStyleSheet("QLabel {color: #CC0000;}");
+		break;
+	default:
+		break;
+	}
+
 #ifdef __APPLE__
 	vis->setAttribute(Qt::WA_LayoutUsesWidgetRect);
 	lock->setAttribute(Qt::WA_LayoutUsesWidgetRect);