Add note about input prompt scaling, and add options to change the key to start and restart searches

[microraptor]

This PR has two commits:

The first one adds only a small note to the readme how the input prompt text scaling can be changed. This setting seems very important for high DPI displays. I have a UHD monitor and scale=1 is not readable and scale=7 comfortable.

I have the keybind to open the script on Ctrl+f instead of f12, because it is more intuitive and better for laptop keyboards and fancy keyboards, which have the F-keys hidden behind a second layer. Changing the keybinding only in input.conf leads to the issue, that restarting a search when a page is already opened is un-intuitive. So the second commit changes only a few lines of the code to allow options to change the f12 default.

I also changed the text Press f12 to enter search query to Enter a new search query, because this text is usually shown after the keybind has been pressed and is therefore misleading.

[CogentRedTester]

I'm fine with this change.

[CogentRedTester]

Most of this PR is actually unnecessary. If we remove these two lines:

Suggested change

	{o.keybind_open_search, "open_search", function() temp:get_input(false) end, {}},
	{o.keybind_open_flag_search, "open_flag_search", function() temp:get_input(true) end, {}}

And change the open_wrapper function to be:

function list_meta:open_wrapper(advanced)
    if self.keyword == nil then self.empty_text = "Press f12 to enter search query" end
    local hidden = self.hidden
    if hidden then self:open() end
    if self.keyword == nil or not hidden then self:get_input(advanced) end
end

Then the standard mpv keybind to open the page will be also be used when the page is open instead of being overridden. Then the keybinds can be changed by using standard input.conf changes.

[CogentRedTester]

Instead of recommending an entry in mpv.conf I would instead prefer to recommend an entry in ~~/script-opts/user_input.conf which would contain scale=1.