ImGui bindings improvements + drag & drop callback

[merlinND]

Depends on nmwsharp/polyscope#365.
Before merging this PR, the deps/polyscope submodule pointer will need to be update to the latest master. (done)

This PR includes:

• Binding for configureImGuiStyleCallback().
• Binding for ImGuiStyle, imgui::GetStyle(), etc.
• Binding for SeparatorText(str).
• Bindings for Image (can use Polyscope quantities to register textures and get their GL index ­— admittedly for advanced users).
• Drag & drop handler: user can register a callback from Python, which gets called with the paths of the files that were dropped onto the window.
• Slice-plane-related bindings

[merlinND]

Hi @nmwsharp,

I have updated this PR to point the Polyscope submodule to nmwsharp/polyscope@dc0c7c2, it's ready for review!