Text input causes panic when dealing with special characters

[pieterdd]

Reproduction scenario:

• Open the widget gallery example
• Go to the 'Input' tab
• Copy this to your clipboard:

tést

• Paste it in the text input field
• Move your cursor to the right of é
• Press the left arrow key once
• Enter any character
• The app will panic:

thread 'main' panicked at /rustc/a28077b28a02b92985b3a3faecf92813155f1ea1\library\alloc\src\string.rs:1538:9:
assertion failed: self.is_char_boundary(idx)

There are several variations to this issue that are likely related, such as typing the character é (on certain keyboard layouts) followed by any character. This scenario also causes a panic. Additionally, when pressing Home and then moving the cursor to the right, you'll notice that the cursor doesn't move from the left to the right of é until you press twice.

Edit: fix at #236

[pieterdd]

#236 has been merged.