Dear ImGui Implementation (#598)

* added imgui as dependency

* imgui renderer/basic input implementation

* imgui: add layers system

Add video info layer to show fps. Press F10 to toggle it.

* imgui: add custom imgui config

* imgui: gamepad capture, stopping propagation

* imgui: changed config & log file path to use portable dir

* videoout: render blank frame when video output is closed

required to render imgui even when game has no video output

- fixed merge compile-error

.gitmodules

@@ -85,3 +85,8 @@
 [submodule "externals/half"]
 	path = externals/half
 	url = https://github.com/ROCm/half.git
+[submodule "externals/dear_imgui"]
+	path = externals/dear_imgui
+	url = https://github.com/shadps4-emu/ext-imgui.git
+	shallow = true
+	branch = docking