Show a warning on the title screen if device has no hardware keyboard or gamepad

[wjt]

Threadbare is not currently playable with a touchscreen. (#806)

I suggest we could show a note/warning to that effect on the title screen if we can detect that the player has neither a keyboard nor a gamepad. Specifically we could check some combination of the following:

• If DisplayServer.has_hardware_keyboard() returns true. Unfortunately this is only implemented for native iOS and Android builds.
• If the web_android or web_ios feature tag is present. It is of course possible to connect a hardware keyboard to an Android or iOS device but we can probably assume they aren't.
• If DisplayServer.is_touchscreen_available() is true.
• If Input.get_connected_joypads() is empty.

Based on some combination of these, show a message on the title screen to the effect of:

You need a gamepad or a keyboard to play Threadbare. You can't play Threadbare with just a touchscreen.

Make it not block the player entering the game in case we have detected wrong.

XP Summary (total 500)

Subdomain	Skill	XP
Engineering: Gameplay & Systems Programming	Problem Solving	75
Engineering: Gameplay & Systems Programming	Creativity & Design Thinking	75
Engineering: Gameplay & Systems Programming	Technical Literacy	150
Game Design: Game Feel & UX	Problem Solving	50
Game Design: Game Feel & UX	Creativity & Design Thinking	75
Game Design: Game Feel & UX	Technical Literacy	75

[manuq]

Make it not block the player entering the game in case we have detected wrong.

This part of the specification is important! The implemented detection may be wrong, considering the complexities.

[wjt]

I noticed one thing that was not covered in #2448 after merging it: there is no check for whether a gamepad/joypad is connected. It is possible to connect a joypad to an Android phone via USB (and presumably Bluetooth but I haven't tested that) and use it in the web browser.

However! I think the slightly misleading warning is probably fine. I don't expect many people would actually do this at this point in development.

[manuq]

Hmm I certainly have a keyboard in this device, but I'm getting the warning. I have 2 actually, one integrated in the laptop and one external.