UI: Rename Default theme to System

The system theme was named Default even though the default theme is Dark.
This addresses that by renaming Default.qss to System.qss. I've made it
backwards compatible so users already using this theme are not affected.

The theme list now shows up as:
-System
-Dark (Default)
-Acri
-Rachni

I have also made it so that you can specify the default theme in the
UI config file.

UI/obs-app.cpp

@@ -54,6 +54,8 @@
 
 #include <iostream>
 
+#include "ui-config.h"
+
 using namespace std;
 
 static log_handler_t def_log_handler;
@@ -423,7 +425,7 @@ bool OBSApp::InitGlobalConfigDefaults()
 
 	if (!config_get_bool(globalConfig, "General", "Pre21Defaults")) {
 		config_set_default_string(globalConfig, "General",
-				"CurrentTheme", "Dark");
+				"CurrentTheme", DEFAULT_THEME);
 	}
 
 	config_set_default_bool(globalConfig, "BasicWindow",
@@ -1026,18 +1028,22 @@ bool OBSApp::InitTheme()
 
 	const char *themeName = config_get_string(globalConfig, "General",
 			"CurrentTheme");
+
+	if (strcmp(themeName, "Default") == 0)
+		themeName = "System";
+
 	if (!themeName) {
 		/* Use deprecated "Theme" value if available */
 		themeName = config_get_string(globalConfig,
 				"General", "Theme");
 		if (!themeName)
-			themeName = "Default";
+			themeName = DEFAULT_THEME;
 	}
 
-	if (strcmp(themeName, "Default") != 0 && SetTheme(themeName))
+	if (strcmp(themeName, DEFAULT_THEME) != 0 && SetTheme(themeName))
 		return true;
 
-	return SetTheme("Default");
+	return SetTheme(DEFAULT_THEME);
 }
 
 OBSApp::OBSApp(int &argc, char **argv, profiler_name_store_t *store)