Modernize input handling

[lapinozz]

Updated the input handling to fix some bugs and to make it more similar to imgui's own backends (mostly looked at the SDL one)

• Use events for touch instead of saving the state and setting it every frame
• Pass events to imgui even if we are not in focus, if we receive the events we should pass them
• Use SFML's function to show the virtual keyboard instead of manually managing it for Android

Fixes #337, #338 and #88

[lapinozz]

After reading previous issues and PRs it seems like the main drawback of using events only for mouse inputs instead of doing global polling is that we might not receive events if there's a transparent window/overlay on top of ours.

IMO that seems like an edge case and the workaround for that is easier/cleaner than the ones for the current issues

see #261 and #227