Blender Git Loki

UI: Tweaks to Add menu in Video Sequencer Editor.

Mainly organization and bringing it closer to the Add menu in the 3D Viewport.

* Icons for most top-level items when available.
* New Transitions menu for Cross, Gamma Cross and Wipe.
* Move Color and Text to top level. These options are used way too often to be in a submenu.
Having a menu with 2 entries defeats the purpose, so I've removed it for now. I checked if
add-ons used it and apparently not. It can be brought back if needed.
* Avoid empty dropdown in Scene/Mask/Clip. Solution is ugly for now and should be improved in C.
* Some effects couldn't be used without 1 or 2 strips selected, now the menu entry is grayed out accordingly.
* Some more icons for other menus.