Fix typeahead backspace cleanup

[dingfeli]

Issue #, if available:
#296 (comment)

Context:

• When an inline session is invoked in the middle of a line and whose suggestion returned as multiple lines (i.e. the suggestion contains line break), the right context on the invocation line is always discarded by the lsp server.
• This intent to discard said right context under the aforementioned condition is denoted visually by a strike through.
• During the preview, said right context is deleted from the user buffer in the editor, but stored in memory.
• Should user decides to accept the suggestion, said right context would be deleted permanently.
• If preview is terminated without acceptance, said right context would once again be appended.

Description of changes:

• Simplifies the logic to capture right context on the same line when starting a typeahead session
• Accordingly simplifies the logic to "return" the right context when said session ends
• Calls redraw on widget in a display thread after having checked for dispose status for safety

Test:
On mac:

• Deleting past inline invocation offset does not cause the subsequent lines to be edited
  

On windows:

• Deleting past inline invocation offset does not cause the subsequent lines to be edited
• NOTE: the indent issue you see here is to be fixed with Ghost text alignment fix windows linux #287
  

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.