VSE: Slip toolbar tool

This PR adds a slip tool in the toolbar with its own custom icon for
applying slips using the mouse. This is useful for e.g. tablets where a
keyboard is not handy and a button would be best to activate the
operator.

There is also a custom cursor that appears when hovering over valid,
slippable strips (and a "stop" icon when the strip cannot be slipped).

Alt may be used in order to ignore slipping connected strips when using
the tool, similar to selection logic. The slip tool only performs its
sole function of slipping and as such does not change the selection
state.

In the future, we can also add "slide" functionality to the same tool,
giving it multiple functions.

Pull Request: https://projects.blender.org/blender/blender/pulls/143513

intern/ghost/GHOST_Types.h

@@ -428,6 +428,7 @@ typedef enum {
   GHOST_kStandardCursorHandClosed,
   GHOST_kStandardCursorHandPoint,
   GHOST_kStandardCursorBlade,
+  GHOST_kStandardCursorSlip,
   GHOST_kStandardCursorCustom,
 
 #define GHOST_kStandardCursorNumCursors (int(GHOST_kStandardCursorCustom) + 1)

intern/ghost/intern/GHOST_SystemWayland.cc

@@ -2893,6 +2893,8 @@ static std::optional<wp_cursor_shape_device_v1_shape> gwl_seat_cursor_find_wl_sh
       return std::nullopt;
     case GHOST_kStandardCursorBlade:
       return std::nullopt;
+    case GHOST_kStandardCursorSlip:
+      return std::nullopt;
     case GHOST_kStandardCursorCustom:
       return std::nullopt;
   }