add loading animation

Create _loader.scss

Update main.scss

Update _base.scss

Update gui.js

Update index.html

removed trailing spaces

removed trailing white spaces for real

scss/main.scss

@@ -4,6 +4,7 @@
 @import '_leaflet';
 @import '_leaflet.label';
 @import '_filters';
+@import '_loader';
 
 $minscreenwidth: 630pt;
 $sidebarwidth: 420pt;