Allow pasting into input popup on tiles build

[Qrox]

Summary

Interface "Allow pasting into input popup on tiles build"

Purpose of change

Allow pasting content from the clipboard into input popups. I imagine this can be useful when you want to search for items etc by name from an external source (item browser, for example). This can also be useful for windows IME users to workaround a SDL bug that makes IME input in the game difficult, by inputing the text into a text editor and then pasting the text into the game instead.

Describe the solution

Test ctrl-v in string_input_popup and insert clipboard text into the string.

Testing

Copied some English and Chinese text from a text editor into a input popup successfully.

Additional context

SDL has a half-done implementation of custom IME candidate list, so neither the system IME candidate list nor the custom one is rendered correctly. If we want this fixed we probably need to compile our own patched SDL, as although a patch has been submitted to the SDL bug tracker two years ago, there doesn't seem to be any intention to merge it or fix the implementation.