Formspec: change the appearance of the cursor on fields and co.

[Desour]

Fixes #8523.

(There's a small bug: If you open a formspec and have your cursor on an element, it will stay normal until you move it away and back.)

To do

This PR is a Ready to Review.

How to test

• Start minetest.
• Join or start a game.
• Wait until you are connected.
• Press esc.
• Hover with you mouse over the text and look at your cursor.

It would be good if users of different operating systems could test this. I'm only able to test with Linux Lubuntu 18.04.

[Desour]

Ok, now it the cursor changes instantly. I overlooked the && delta >= m_tooltip_show_delay and instead looked for problems in irrlicht. >_<

[SmallJoker]

Works nicely.

[Desour]

Thanks @SmallJoker!

But note that, according to irrlicht docs, this probably doesn't work for mac users.

[Desour]

Later in another PR, other cursor icons could optionally be used for other elements, eg. hand for buttons, a little bit related: #8901.

[SmallJoker]

@DS-Minetest Needs rebase and someone else who's keen to review these few changed lines.

[Desour]

Rebased.

[rubenwardy]

@SmallJoker what platform did you test on? This is the sort of thing that could cause issues due to dodgy implementations on other platforms

[SmallJoker]

@rubenwardy Linux, Xubuntu 18.04. It's really not a good comparison among different platforms but Minetest surely isn't the first app that makes use of different cursors (in Irrlicht).

[SmallJoker]

http://irc.minetest.net/minetest-dev/2020-01-11#i_5631064