Support Vertical Mode and Dark Mode

Improvement Highlights

• 2 new settings style/candidate_list_layout and style/text_orientation the first takes linear or stacked, the latter takes horizontal or vertical. vertical adopts traditional Chinese vertical layout. There are 2 × 2 = 4 layouts in total now.
• Squirrel now supports setting border_color, preedit_back_color, base_offset.
• hilited_back_color now is displayed full size, the same as hilited_candidate_back_color, and benefits from hilited_corner_radius as well.
• May now use style/color_scheme_dark to specify a theme for system dark mode, will automatically change.
• Horizontal mode (actually all modes) supports arbitrary candidate number, if too long a line, will break line, and highlights will follow accordingly. Preedit line will also behave similarly.
• Round corners are now drawn with cubic bezier lines, so that roundness transits smoothly.
• Add color_space color option to explicitly use Display P3 color gamut.