TextInput cursor visual offset when pasting

[macarooni-man]

For text boxes that have a character limit, pasting will move the cursor even if no text was added. This will require the following fixes:

• Add a default "character limit" attribute to the custom BaseInput class
• Once the character limit is added, use that in both insert_text() to prevent text from being inserted (like it is now) and figure out what function Kivy uses to move the cursor, and prevent that from running when the character limit is met or exceeded