SDL: Add text composition event

[y5nw]

Add compact, short information about your PR for easier understanding:

• Goal of the PR
  Add text composition/edit events to Luanti.
• Does it resolve any reported issue?
  No.
• Does this relate to a goal in the roadmap?
  No.
• If not a bug fix, why is this PR needed? What usecases does it solve?
  IMO this is a minor quality of life improvement as some dead/compose keys do not show the status of text composition otherwise.

To do

This PR is a Work in Progress.

• Introduce text composition event to Irrlicht.
• Correctly handle text coposition:
  • Restore selected text when composition is canceled.
  • Correctly handle overwrite mode.
  • Correctly place the cursor within text composition.
• Make UI elements use text composition:
  • CGUIEditBox
  • Chat console
• Code refactoring?

How to test

Enter text using an IME / compose key / dead key and observe that "partial" input is also shown in the input box.