Scale fonts context menu (#246)

* Retrieve scale plugin from lite-plugins

* New implementation of scale plugin and font C API

Introduce two new C API functions, renderer.font.get_size and set_size
respectively to get the font size and to set the size to a new value.

Using these functions we don't need to know the name of the font but
we can just change their size.

Adapt the scale plugin to use the new C API function with minor adaptations
in the logic.

Use smaller step to scale fonts.

Rename font_desc_free function, previous name was misleading as only the cached
resources are freed.

* Add contextmenu plugin from takase

From https://github.com/takase1121/lite-contextmenu

Adapted to show font scaling commands and find/replace commands.

i#	testing.lua

* Fix the cursor flickering with contextmenu

To avoid flickering of the cursor when using the context menu
we add a new function `core.request_cursor` that just take note
of the cursor requested.

The cursor will be actually changed only in root_view:draw() method
only when all the drawing operations are done. This means the cursor
will be changed only once per frame and only the most recent cursor
change request will take effect.

* Remove unneeded scale plugin return functions

data/core/init.lua

@@ -1073,6 +1073,11 @@ function core.blink_reset()
 end
 
 
+function core.request_cursor(value)
+  core.cursor_change_req = value
+end
+
+
 function core.on_error(err)
   -- write error to file
   local fp = io.open(USERDIR .. "/error.txt", "wb")

data/core/rootview.lua

@@ -783,7 +783,7 @@ end
 
 function RootView:on_mouse_moved(x, y, dx, dy)
   if core.active_view == core.nag_view then
-    system.set_cursor("arrow")
+    core.request_cursor("arrow")
     core.active_view:on_mouse_moved(x, y, dx, dy)
     return
   end
@@ -808,14 +808,14 @@ function RootView:on_mouse_moved(x, y, dx, dy)
   local div = self.root_node:get_divider_overlapping_point(x, y)
   local tab_index = node and node:get_tab_overlapping_point(x, y)
   if node and node:get_scroll_button_index(x, y) then
-    system.set_cursor("arrow")
+    core.request_cursor("arrow")
   elseif div then
     local axis = (div.type == "hsplit" and "x" or "y")
     if div.a:is_resizable(axis) and div.b:is_resizable(axis) then
-      system.set_cursor(div.type == "hsplit" and "sizeh" or "sizev")
+      core.request_cursor(div.type == "hsplit" and "sizeh" or "sizev")
     end
   elseif tab_index then
-    system.set_cursor("arrow")
+    core.request_cursor("arrow")
     if self.dragged_node and self.dragged_node ~= tab_index then
       local tab = node.views[self.dragged_node]
       table.remove(node.views, self.dragged_node)
@@ -823,7 +823,7 @@ function RootView:on_mouse_moved(x, y, dx, dy)
       self.dragged_node = tab_index
     end
   else
-    system.set_cursor(node.active_view.cursor)
+    core.request_cursor(node.active_view.cursor)
   end
 end
 
@@ -858,6 +858,10 @@ function RootView:draw()
     local t = table.remove(self.deferred_draws)
     t.fn(table.unpack(t))
   end
+  if core.cursor_change_req then
+    system.set_cursor(core.cursor_change_req)
+    core.cursor_change_req = nil
+  end
 end