Steam + Gamepad: Use Steam's on-screen virtual keyboard

[ChrisNZL]

Currently in 0.4 with Steam, when trying to edit a player's name or fill out the feedback form, you have to use a physical keyboard to enter text.

todo:

• [x] Allow Steam to open an on-screen virtual keyboard for controller input
• https://partner.steamgames.com/doc/features/steam_controller/getting_started_for_devs#6

On-screen text input is not technically part of the ISteamInput, but is found instead in ISteamUtils.

Some quick references:

• ShowGamepadTextInput
• GetEnteredGamepadTextLength
• GetEnteredGamepadTextInput

https://partner.steamgames.com/doc/api/ISteamUtils#IsSteamInBigPictureMode

Scripting Define Symbol: ONSCREEN_KEYBOARD

app.ShowTouchScreenKeyboard(

Called from:
- InputPrompt.ShowPromptForCustomString
- FeedbackWindow.ShowOnscreenKeyboard

bool IsSteamInBigPictureMode();

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• [x] Mark controller support as "full controller support" on Steam's store page backend

[ChrisNZL]

0.4.0f:

• Added Steam's "Gamepad Text" virtual on-screen keyboard for use with simple input boxes in-game (editing Player Name, editing Password for online games).
• Does not support the Feedback Form due to too many buttons and navigation stuff, it's more of a debug thing anyhow rather than a main feature.