Extract calculating font sizes to calc_font_sizes() method.

diff --git a/qwerty.py b/qwerty.py
index 1c92641..aec4c1e 100755
--- a/qwerty.py
+++ b/qwerty.py
@@ -558,9 +558,6 @@ class KeyEditDialog(object):
 class GUI(object):
     def __init__(self):
         global sections
-        global LABEL_FONT_SIZE
-        global DESC_FONT_SIZE
-        global KEY_FONT_SIZE

         self.buttons = {}
         self.sbuttons = []
@@ -581,9 +578,7 @@ class GUI(object):
             self.width, self.height = self.window.get_size()
         self.window.set_decorated(decorate)

-        LABEL_FONT_SIZE = LABEL_FONT_SIZE_COEF * self.height
-        DESC_FONT_SIZE  = DESC_FONT_SIZE_COEF  * self.height
-        KEY_FONT_SIZE   = KEY_FONT_SIZE_COEF   * self.height
+        self.calc_font_sizes()

         for c in ALPHABET:
             self.buttons[c] = SuperButton(c)
@@ -640,6 +635,16 @@ class GUI(object):

         self.collect_windows()

+    def calc_font_sizes(self):
+        global LABEL_FONT_SIZE
+        global DESC_FONT_SIZE
+        global KEY_FONT_SIZE
+
+        LABEL_FONT_SIZE = LABEL_FONT_SIZE_COEF * self.height
+        DESC_FONT_SIZE  = DESC_FONT_SIZE_COEF  * self.height
+        KEY_FONT_SIZE   = KEY_FONT_SIZE_COEF   * self.height
+
+
     def convert_class(self,clss):
         if isinstance(clss, tuple):
             return clss