[Frontend] Significant mods to main layout strategy

open #90
IN-PROGRESS
To support desired edge-to-edge styling of Inspector pane;
Removed superflous .scss file;
Enhancements to splitter .scss to allow flush edging and
edge shadows;

platform/commonUI/general/res/sass/_main.scss

@@ -70,7 +70,6 @@
 @import "lists/tabular";
 @import "plots/plots-main";
 @import "iframe";
-@import "hide-non-functional";
 @import "views";
 @import "items/item";
 @import "mobile/item";