Use inplace_merge for sorting matches in region highlighter
RegionHighlighter: Find the lines to be reparsed using the changes
Word completion use a WordDB stored as a buffer value.
FunctionGroup: fix incorrect parameter for operator()
only highlight actually displayed regions
Pass a HighlightFlags parameter to highlighters

When only highlighting for finding character position on screen
pass HighlightFlags::MoveOnly so that non moving highlighters do
not need to run.
RegionHighlighter: correct coordinates that are one past end of line

As region highlighters store per lines matches, end of line ones
are not valid buffer coordinates and must be corrected as begining
of next line.
Use regions for string highlighting in kakrc files
WordDB: use an ordered map for storing words

This way we can use lower_bound to find the first prefix match
in logarithm time and we know all other prefix matches will
follow.
Add a show_matching highlighter
Fix adapt_view_pos when the cursor is not on a display atom by itself