Text shaping using Harfbuzz - New mirror() method - New AES-256 encryption - Several bugfixes

Added

• FPDF.set_text_shaping(): new method to perform text shaping using Harfbuzz - documentation - thanks to @andersonhc in PR #820
• FPDF.mirror() - New method: documentation page - Contributed by @sebastiantia
• FPDF.table(): new optional parameters gutter_height, gutter_width and wrapmode. Links can also be added to cells by passing a link parameter to Row.cell()
• FPDF.multi_cell(): has a new optional center parameter to position the cell horizontally at the center of the page
• New AES-256 encryption: documentation - thanks to @andersonhc in PR #872
• Added tutorial in Khmer language: ភាសខ្មែរ - thanks to @kuth-chi
• Added tutorial in 日本語 - thanks to @alcnaka
• Better documentation & errors when facing HTML rendering limitations for <table> tags: https://pyfpdf.github.io/fpdf2/HTML.html

Fixed

• FPDF.table(): the colspan setting has been fixed - documentation
• FPDF.image(): allowing images path starting with data to be passed as input
• text overflow is better handled by FPDF.write() & FPDF.write_html() - cf. issue #847
• the initial text color is preserved when using FPDF.write_html() - cf. issue #846
• PDF metadata not encrypted - cf. issue #865
• handle superscript and subscript correctly when rendering TextLine- thanks to @Tolker-KU - cf. Pull Request #862
• make sure warnings always point to the users code - cf. Pull request #869

Deprecated

• the center optional parameter of FPDF.cell() is no more deprecated, as it allows for horizontal positioning, which is different from text alignment control with align="C"