Refactor DynamicRegexHighlighter, search hl uses Search colalias

fixes #122
Fix show_matching highlighter when opening char is the first buffer char

Fixes #119
Fix :delbuf! not forcing buffer deletion

Fixes #116
Fix RegexColorizer cache handling
utf8::is_character_start takes directly the char value
Use a plain SelectionList for Context, remove DynamicSelectionList
Remove BufferChangeListener
Iterate in reversed order on selections when modifing buffer

This way, update only needs to be called once everything is done
as we always modify after the next selection to be used.
Remove unneeded vector for computing line modifications
replace LineChangeWatcher with a free function compute_line_modifications
Fix gdb pretty printer
read all available text in fifo each time the fd is signaled
DisplayAtom::content returns a StringView
set Coords/Counts methods as always_inline

They are well tested, and we never want to step into them when debugging
Merge branch 'master' into remove-buffer-change-listener

Conflicts:
	src/highlighters.cc