Pause the game (with g_stopTime) when the settings menu is opened
while ingame, unpause it when it's closed.
If the menu is open while ingame and an ImGui window has focus,
the mouse cursor is shown. If the player clicks outside an ImGui window,
it gets unfocused and the cursor is hidden and the player can look
around. Pressing F10 (or whatever key is bound to "dhewm3Settings")
will give focus back to an open ImGui window, pressing it again then
will close the settings window, pressing it once again afterwards will
open the settings window again.
handleMouseGrab() (in sys/events.cpp) now checks if sys_imgui thinks
that a cursor should be shown (via D3::ImGuiHooks::ShouldShowCursor())
and if so, shows it and ungrabs the mouse. This, together with
D3::ImGuiHooks::NewFrame() checking ShouldShowCursor() to (unset)
ImGuiConfigFlags_NoMouseCursorChange, should prevent flickering cursor
problems that sometimes occurred when ImGui's SDL2 backend and dhewm3
disagreed on whether the cursor should be visible.
