TextBoxWidget: allow showing bits of text in bold

Allow for embedding "tags" (invalid Unicode codepoints)
in the text string to trigger some text formatting:
for now only bolding some parts of text is possible.

Use it with fulltext search "all results" to highlight the
matched word (instead of the previously used brackets).

frontend/document/koptinterface.lua

@@ -12,6 +12,7 @@ local FFIUtil = require("ffi/util")
 local Geom = require("ui/geometry")
 local KOPTContext = require("ffi/koptcontext")
 local Persist = require("persist")
+local TextBoxWidget = require("ui/widget/textboxwidget")
 local TileCacheItem = require("document/tilecacheitem")
 local Utf8Proc = require("ffi/utf8proc")
 local logger = require("logger")
@@ -1510,15 +1511,18 @@ function KoptInterface:findAllText(doc, pattern, case_insensitive, nb_context_wo
                         i_next, j_next = i, j
                     end
                 end
-                text = "【" .. table.concat(text, " ") .. "】"
+                -- Make this word bolder, using Poor Text Formatting provided by TextBoxWidget
+                -- (we know this text ends up in a TextBoxWidget).
+                text = TextBoxWidget.PTF_BOLD_START .. table.concat(text, " ") .. TextBoxWidget.PTF_BOLD_END
                 local prev_text = get_prev_text(text_boxes, i_prev, j_prev, nb_context_words)
                 if prev_text then
-                    text = prev_text .. text
+                    text = prev_text .. " " .. text
                 end
                 local next_text = get_next_text(text_boxes, i_next, j_next, nb_context_words)
                 if next_text then
-                    text = text .. next_text
+                    text = text .. " " .. next_text
                 end
+                text = TextBoxWidget.PTF_HEADER .. text -- enable handling of our bold tags
                 res_item.text = text
                 table.insert(res, res_item)
                 if #res == max_hits then