Add Padding to the TextBox

[hojjatabdollahi]

This adds text margins to the TextBox and makes sure text selection works as expected. I'm leaving this PR here in case you guys have any suggestions.

• [x] left_margin and right_margin.
• [x] Fix the issue with text selection if you start dragging from the side margins.
• [x] top_margin and bottom_margin.
• [x] Text selection has an issue (in LTR only) when selecting from the beginning of the line until the left half of the first glyph. It erroneously selects the whole line.
• [x] Text selection has an issue when selecting text in a RTL paragraph that contains LTR segments. Specifically when the LTR segment falls on a separate line. The last letter in the LTR segment can't be selected.