Use ImGuiKey_Menu and Shift + F10 to invoke the context menu via keyboard

[exelix11]

I have been experimenting with this for better keyboard accessibility. I'd suggest allowing the use of ImGuiKey_Menu for this since that is the exact reason this key exists.

This issue was also was discussed in #8803 and it was mentioned that the windows convention for this is Shift + F10. I also implemented it as an alternative for keyboards that don't have ImGuiKey_Menu.

I'm not familiar enough with ImGui internals to know if this causes any issues so consider this mostly a draft to discuss potential issues or a better implementation

Most content in the demo window works with these changes, but there are a couple of issues still open:

• Popups attached to non-selectable items can't be opened since the nav cursor can't highlight those UI components. Example:

  imgui/imgui_demo.cpp

  Lines 5412 to 5413 in ba84d2d

  	ImGui::Text("(2) Or right-click this text");
  	ImGui::OpenPopupOnItemClick("my popup", ImGuiPopupFlags_MouseButtonRight);

• BeginPopupContextWindow opens the context menu near the mouse cursor rather than next to the nav cursor, this is not an issue but just an annoyance. One such example is the asset browser

  imgui/imgui_demo.cpp

  Lines 10996 to 10998 in ba84d2d

  	if (ImGui::BeginPopupContextWindow())
  	{
  	ImGui::Text("Selection: %d items", Selection.Size);

• What about items that have different popups depending on which button is pressed? I didn't find any such case in the demo window and I'd argue this is an anti-pattern since there is no meaningful way to make it keyboard-accessible. It's not something that should be encouraged.
• Lots of code is repeated, maybe it needs an helper function to check for the keypresses?

[ocornut]

This is now merged with changes. See #8803 (comment) for details. Thank you!