[UI] Add detail bar of selected asset.

[liubai01]

This is a feature request for a detailed info bar of an asset.

Background

• Asset explorer helps users organize resources in the filesystem.
• Current implementation of asset icon in 4f19414 is in CoreAssetWidget.cpp.
• The GUI of the editor is based on Dear ImGui.

Goal

• Define a pointer of the selected asset.
• When the user does a single mouse left click over an icon, the asset is selected (not opened.).
• Show the asset's path(full name) in an info bar.

Example of unity:

References and guidance

For a quick start of getting handy on Dear imGUI, refer to imgui_demo.cpp.

A quick introduction to construct a widget could refer to this video (in Mandarin Chinese).

You could uncomment ImGui::ShowDemoWindow(); to show example of widgets of dearImGui in CoreApp.cpp

void CoreApp::BeforeUpdate()
{
    mGUIManager->Update();
    //ImGui::ShowDemoWindow();
}

For example, the logic of a folder.

...
} else if (assetNode->GetAssetType() == BaseAssetType::ASSET_FOLDER) {
				ImVec2 pos = ImGui::GetCursorPos();
				if (ImGui::Selectable(assetNode->mID.c_str(), false, ImGuiSelectableFlags_AllowDoubleClick, ImVec2(iconSize, iconSize)))
				{
					ImGuiIO& io = ImGui::GetIO();
					if (io.MouseDoubleClicked[0])
					{
						mNowSelectedAssetNodeDir = assetNode.get();
					}
				}

				// manually set a padding of 20.0 pixel (TBD: remove this magic number in the future)
				ImGui::SetCursorPos(ImVec2(pos.x + 20.0f, pos.y + 20.0f));
				BaseResourceImage* img = mResourceManager->LoadByURL<BaseResourceImage>("EditorAsset\\icon\\folder.png");
				ImGui::Image(img->GetImGuiRuntimeID(), ImVec2(iconSize - 40.0f, iconSize - 40.0f));

				ImGui::SetCursorPos(ImVec2(pos.x, pos.y + iconSize));
...