Hey,

After using the very limited styling options for the old drag-drop target rect for as long as I can remember - I decided it was time to finally add some semi-useful options so I can achieve the look I wanted.

This PR adds three new style variables and a color. These new variables/color are defaulted to their old, previously hard-coded values so nothing will change visually unless you explicitly change these new options.

Here's a quick look at what is possible, and the code I'm using to achieve it

Before

After

style->DragDropTargetRectRounding      = style->FrameRounding;
style->DragDropTargetRectExpansionSize = 1.0f;
style->DragDropTargetRectLineThickness = 2.0f;

colors[ImGuiCol_DragDropTarget]   = ImVec4(0.26f, 0.59f, 0.98f, 1.00f);
colors[ImGuiCol_DragDropTargetBg] = ImVec4(0.26f, 0.59f, 0.98f, 0.25f);

Happy to take feedback and make changes on the naming of these variables.