Formspec: Implement Ctrl+Shift+Left/Right text selection

[SmallJoker]

This is a quality-of-life improvement to edit text more easily. Here I tried to mimic the logic of Firefox.

This does NOT implement text selection of single-line input boxes. This shall be addressed by merging GUIEditBox into CGUIEditBox in a future PR. (yes we have some duplicate code)

To do

This PR is Ready for Review.

How to test

1. Either use the server list description box to navigate, or edit mainmenu/tab_local.lua:

	elseif is_world_selected then
		retval = retval ..
				"button[10.1875,5.925;4.9375,0.8;play;" .. fgettext("Play Game") .. "]"
	end

local text = [[
01""field[0,".. y .. ";3,0.75;te_serveraddr;"
02      download .doc file (zipped)
03https://a.domain.tld/word-bar/aaa.jpeg?key_text=value%20what%20ever
04 AAA000... AA00.. A0. A. .A .A0
05
06頂とょんき回独サ午映就
07Chuchichäschtli
08
09
10
11
12
13
]]
	--text = "sasasas aaaa dd"

	retval = "textarea[1,1;8,3;mytext;Title Name;" .. core.formspec_escape(text) .. "]"

	return retval

2. Use the Shift+Left/Right, Ctrl+Shift+Left/Right key combinations to select text.
3. It must feel rather consistent with other apps.

[SmallJoker]

(The build shows up as fail because the macOS dependencies could not be installed properly.)

[sfan5]

works