dialog: adding maxWidth to SingleTextInputDialog

[vladarama]

What it does

This PR adds a maxWidth attribute to SingleTextInputDialogs in order to fix a styling issue that would occur when the input validation error message would get too long. This would cause the dialog to extend indefinitely. This fix adds a max-width of 400 px to the dialogs to make their styling more user-friendly. The modified dialogs include: New File, New Folder, Rename and EditKeybinding dialogs.

Before:

After:

How to test

1. Open Theia application
2. Try causing errors while renaming files, creating new files/folders, modifying keybindings
3. The dialog should have a max-width: 400px css style
4. Long error messages should not extend the dialog indefinitely

Easy errors to test include: creating a file/folder with no name or a duplicate name,
renaming a file to the same name, creating empty key bindings ...

Should be merged after #12585

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Alright, looks good to me 👍