Gpencil: gradient tool added to weight paint tools

This patch adds the gradient tool to the weight paint tools in Grease Pencil.
The gradient tool follows the same logic as the weight gradient tool on
3D meshes. The gradient is drawn interactively in the viewport, shown by
a dotted line and the vertex weight colors changing on the fly.

The user can:
- choose between a linear and radial gradient
- switch between adding/subtracting weight with shortcut 'D'

@@ -160,1 +161,4 @@
             settings = tool_settings.gpencil_weight_paint
+            # The brush of the gradient tool isn't automatically activated
+            # by the window manager, so we select it here manually.
+            tool = context.workspace.tools.from_space_view3d_mode(context.mode, create=False)

@@ -1520,0 +1524,4 @@
+   * by the window manager, so we select it here manually.
+   */
+  bToolRef *tref = WM_toolsystem_ref_from_context(C);
+  if (tref && STREQ(tref->idname, "builtin.gradient")) {

@@ -1684,0 +2020,4 @@
+  tool_data->line_segment_len_sq = dot_v2v2(tool_data->line_segment, tool_data->line_segment);
+  tool_data->radius = sqrtf(tool_data->line_segment_len_sq);
+  if (tool_data->line_segment_len_sq == 0.0f) {
+    goto finally;