Internal refactoring (#212)

* Refactored CSS properties

All CSS related properties are moved into the separate class css_properties.
Getters and setters are removed from classes element and html_tag.
Access to the css_properties rleased via css() [ro] and css_w() [rw] methods

* fix: el_text don't have to copy all css properties from parent

* Refactored rendering code

* Added flex and inline_flex values for display css property

* Implementing box generation

https://www.w3.org/TR/CSS22/visuren.html#box-gen

* Split inlines on block box inside

* Split parsing and rendering trees.

* Fixed some bugs

* Fixed: impossible to click urls on Obama wiki's toc

* Make element::get_placement work again

* Fixed: incorrect rendering table captions

* find_styles_changes function returned to the element class

* set parent correctly during render items split

* fixed urls on https://en.cppreference.com/w/cpp/container/vector

* fixed rendering blocks with width in percents

Example:
https://web.archive.org/web/20110101155107/http://www.unicode.org/
Issue #208

* Fixed placement of blocks with "overflow: hidden" with floating boxes.

* refactoring of rendering block

* Selectors :before and :after returned back with fixed behaviour.

* fixed render_item::is_last_child_inline

* fixed: text inside nested inlines has extra paddings/margins

* fixed documet test

Author: tordex

documentTest.cpp

@@ -37,7 +37,7 @@ TEST(DocumentTest, CvtUnits) {
   container_test container;
   litehtml::document::ptr doc = std::make_shared<litehtml::document>(&container, nullptr);
   bool is_percent;
-  doc->cvt_units(_t(""), 10, &is_percent);
+  doc->to_pixels(_t(""), 10, &is_percent);
   css_length c;
   c.fromString(_t("10%")), doc->cvt_units(c, 10, 100);
   c.fromString(_t("10em")), doc->cvt_units(c, 10, 100);