Support controllers

[tooda02]

Although Facepunch supports a few Steam Input functions, it is missing several essential ones and cannot be used in its current state to implement Steam Input (controller) support. This PR adds:

• Init()
  Required to set up the SteamInput interface

• GetAnalogActionGlyph()
  Gets a glyph for the specified analog (joystick) action

• GetAnalogActionDescription()
  Gets the localized description for the specified analog action

• GetAnalogActionOrigins()
  Return an array of all origins associated with the specified analog action

• GetDigitalActionDescription()
  Gets the localized description for the specified digital (button) action

• GetDigitalActionOrigins()
  Return an array of all origins associated with the specified digital action

• GetPngActionGlyphForOrigin()
  Return an absolute path to the PNG image glyph for the specified action origin

• InputActionOrigin
  An existing internal enum made public to allow its use by the application using Facepunch

• ShowBindingPanel()
  Displays the Steam View Controller/Edit Controller form

• STEAM_INPUT_MAX_ORIGINS
  An integer constant containing the maximum size of the array returned by the two Get...ActionOrigins() functions

In addition, the PR adds the following SteamUtils function and action supporting the on-screen keyboard:

• ShowFloatingGamepadTextInput()
  Displays a full keyboard allowing the player to type into the currently-selected field

• OnFloatingGamepadTextInputDismissed
  Action invoked when the player closes the on-screen keyboard

This update is used in the controller support of the publicly available game Stinky and Loof in Wonderland.