InputDialog: add search (#7701)

Searches for a string in the edited text.
Available in the Text editor and other input dialogs with the navigation bar enabled.

Find first searches from the beginning of the text.
Find next searches from the next to cursor position, used for continious search.
By now, the Search input window is closed after the search. You need to press the Find button again for continious search, the search string is kept in the input.
Is it better to keep the dialog open for the comfortable continious search? And close it with the Cancel only?

Case insensitive. Cursor jumps to the beginning of the found string.
Notifications are shown for better results visibility.

Unfortunately, violated our standartization to "search", couldn't invent better short wordings.

frontend/ui/widget/inputtext.lua

@@ -533,6 +533,29 @@ function InputText:getLineCharPos(line_num)
     return char_pos
 end
 
+--- Search for a string.
+-- if start_pos not set, starts a search from the next to cursor position
+-- returns first found position or 0 if not found
+function InputText:searchString(str, start_pos)
+    local str_len = string.len(str)
+    local char_pos, found = 0, 0
+    start_pos = start_pos and (start_pos - 1) or self.charpos
+    for i = start_pos, #self.charlist - str_len do
+        for j = 1, str_len do
+            if string.lower(self.charlist[i + j]) ~= string.lower(string.sub(str, j, j)) then
+                found = 0
+                break
+            end
+            found = found + 1
+        end
+        if found == str_len then
+            char_pos = i + 1
+            break
+        end
+    end
+    return char_pos
+end
+
 function InputText:addChars(chars)
     if not chars then
         -- VirtualKeyboard:addChar(key) gave us 'nil' once (?!)