Enable all features on Smartphones - don't require alt-left click (aka opt-left click)

[david-a-wheeler]

Metamath-lamp is surprisingly functional on smartphones. However, there is no "alt-left click" on typical smartphones. I propose adding some mechanism so that users who can't (easily) do alt-left click can still access all of metamath-lamp's functionality.

I propose adding two more buttons on the toolbar:

• "S" - Select statement fragments. When this is selected, the cursor changes, and you can pick a statement, which immediately opens like alt-left click & lets you keep selecting statement fragments. Use circled X, or this "S" again, to leave the mode.
• "H" - "Hypothesis Toggle" - Only works when 1+ statements are selected; those statements are toggled between "hypothesis" or not.

Feel free to implement it some better way. At the least, being able to create hypotheses is key for creating proofs on smartphones, but simplifying statement fragment handling is also very nice on smartphones.

The only other issue on smartphones that I found is that smartphone touchscreens don't have "hover". To address that, see #23. In short, long-downpress could also show the hover text (but make sure the pressing finger doesn't cover the text!).

[david-a-wheeler]

Here's another idea to meet this goal: interpret a "long click" as alt-left click instead of left-click. A "long click" is a downpress, held for a long time (say 1.5seconds or more), then an uppress.

As far as I know React doesn't directly support longpress, but it doesn't appear to be hard to implement. Here's a discussion and sample code:
https://stackoverflow.com/questions/48048957/react-long-press-event

The "time for long press" could be a configurable setting.

Even if this is done, a "Hypothesis toggle" button might still be a good idea, because that is far more discoverable (it would only work if one or more statements are selected).

[david-a-wheeler]

I think this is ready-on-dev, yes?

[expln]

Yes, that's ready-on-dev. Just for the future reference - #97