VirtualKeyboard: Revamp visibility handling (#10852)

Move as much of the state tracking as possible inside VirtualKeyboard itself.
InputDialog unfortunately needs an internal tracking of this state because it needs to know about it *before* the VK is shown, so we have to keep a bit of duplication in there, although we do try much harder to keep everything in sync (at least at function call edges), and to keep the damage contained to, essentially, the toggle button's handler.

(Followup to #10803 & #10850)

plugins/texteditor.koplugin/main.lua

@@ -545,13 +545,13 @@ function TextEditor:editFile(file_path, readonly)
         cursor_at_end = false,
         readonly = readonly,
         add_nav_bar = true,
-        keyboard_hidden = not self.show_keyboard_on_start,
+        keyboard_visible = self.show_keyboard_on_start, -- InputDialog will enforce false if readonly
         scroll_by_pan = true,
         buttons = {buttons_first_row},
-        -- Set/save view and cursor position callback
+        -- Store/retrieve view and cursor position callback
         view_pos_callback = function(top_line_num, charpos)
-            -- This same callback is called with no argument to get initial position,
-            -- and with arguments to give back final position when closed.
+            -- This same callback is called with no arguments on init to retrieve the stored initial position,
+            -- and with arguments to store the final position on close.
             if top_line_num and charpos then
                 self.last_view_pos[file_path] = {top_line_num, charpos}
             else
@@ -572,7 +572,7 @@ function TextEditor:editFile(file_path, readonly)
         end,
         -- File saving callback
         save_callback = function(content, closing) -- Will add Save/Close buttons
-            if self.readonly then
+            if readonly then
                 -- We shouldn't be called if read-only, but just in case
                 return false, _("File is read only")
             end
@@ -641,8 +641,10 @@ Do you want to keep this file as empty, or do you prefer to delete it?
 
     }
     UIManager:show(input)
-    input:onShowKeyboard()
-    -- Note about self.readonly:
+    if self.show_keyboard_on_start and not readonly then
+        input:onShowKeyboard()
+    end
+    -- Note about readonly:
     -- We might have liked to still show keyboard even if readonly, just
     -- to use the arrow keys for line by line scrolling with cursor.
     -- But it's easier to just let InputDialog and InputText do their