Cleanup in undo in TextEdit and LineEdit

[TheSofox]

In both TextEdit and LineEdit, the Undo and Redo functions seemed to check for themselves whether there was an Undo/Redo available rather than using the existing has_undo or has_redo functions. I decided to make them use them to remove repetition.

Only exception was the TextEdit Undo function. I noticed its check for an Undo was slightly different from the has_undo function. I decided to leave it alone.

[Repiteo]

Thanks!