Tab view (#2197)

* first cut, working for objects with explicit height

* working for all objects by setting min-height explicitly to 70vh

* add composition listeners

* Tabs view WIP

- Markup and CSS BEMized;
- Stubbed in type icon in tab elements;
- TODO: refine styling on tabs;

* Tabs view WIP

- Layout enhancements;

* add types and header

* remove static icon-layout class

* move Tabs into its own plugin folder:

* simplify on composition add listener callback

* fix icon rendering

* add document dragstart and dragend listeners, with v-if div for drop target

* remove third argument from document listeners, move drop target to tab container

* Sanding and shimming on Tabs View

- WIP

* Tabs View styling

- Shippable to Deep;
- Added new c-drop-hint element in _controls.scss;
- Added new theme constants;
- Added 'empty' tabs view message;
- TODOs: add listener for dragover event for is-mouse-over styling,
integrate forthcoming changes to symbolsfont;

* add is-mouse-over class when drag enters dropHint div, add and remove listeners

* .c-drop-hint styling refined

- Refined hover effects;
- Bg icon instead of glyph character;
- TODOs: Change $bg-icon to plus sign;

* fix bug regarding persisting drop-hint styling even after drop is ended

index.html

@@ -77,6 +77,7 @@
         openmct.install(openmct.plugins.SummaryWidget());
         openmct.install(openmct.plugins.Notebook());
         openmct.install(openmct.plugins.FolderView());
+        openmct.install(openmct.plugins.Tabs());
         openmct.time.clock('local', {start: -THIRTY_MINUTES, end: 0});
         openmct.time.timeSystem('utc');
         openmct.start();