Textblock: Fix padding computation (part 1)

Textblock: Fix padding computation (part 1)

Track padding per paragraph, since this is how it is computed.

Problem before this patch:
- If markup text is changed, padding may grow, and the layout is updated (good)
- If the UI itself needs a relayout, the old padding value is NOT reused,
so style paddings will reset the padding to 0.

Test protocol:
- Set some text with style=glow. The whole object should have padding 2,2,2,2
- Relayout the UI, the whole object will have padding 0,0,0,0 (should be 2,2,2,2)