Clean up char atlas constant imports/exports

- Ensures that everything is only defined in one place, and everything
  imports from that place.
- Moves CHAR_ATLAS_CELL_SPACING into a subdirectory of shared/ so that
  CharAtlasGenerator can pull from it.

This addresses the comments on
https://github.com/xtermjs/xterm.js/pull/1307/files/454fd4bfb1ab5c9ece

src/renderer/LinkRenderLayer.ts

@@ -7,7 +7,8 @@ import { ILinkHoverEvent, ITerminal, ILinkifierAccessor, IBuffer, ICharMeasure,
 import { CHAR_DATA_ATTR_INDEX } from '../Buffer';
 import { GridCache } from './GridCache';
 import { FLAGS, IColorSet, IRenderDimensions } from './Types';
-import { BaseRenderLayer, INVERTED_DEFAULT_COLOR } from './BaseRenderLayer';
+import { INVERTED_DEFAULT_COLOR } from './atlas/Types';
+import { BaseRenderLayer } from './BaseRenderLayer';
 
 export class LinkRenderLayer extends BaseRenderLayer {
   private _state: ILinkHoverEvent = null;