E-book viewer: Add a shortcut ctrl+0 to restore default font size

Fixes #1925294 [[Enhancement] E-book viewer: there should be a tool bar icon/keyboard shortcut for setting font zoom to 100%](https://bugs.launchpad.net/calibre/+bug/1925294)

src/pyj/read_book/prefs/font_size.pyj

@@ -10,6 +10,7 @@
 from dom import add_extra_css, rule, unique_id
 from read_book.globals import ui_operations
 from widgets import create_button
+from session import defaults
 
 CONTAINER = unique_id('font-size-prefs')
 MIN_FONT_SIZE = 8
@@ -56,6 +57,10 @@ def change_font_size_by(frac):
     change_font_size(nsz)
 
 
+def restore_default_font_size():
+    change_font_size(defaults.base_font_size)
+
+
 def display_changed_font_size(sz):
     sz = max(MIN_FONT_SIZE, min(int(sz), MAX_FONT_SIZE))
     sz += ''

src/pyj/read_book/shortcuts.pyj

@@ -248,6 +248,12 @@ def common_shortcuts():  # {{{
         _('Decrease font size'),
     ),
 
+    'default_font_size': desc(
+        "Ctrl+0",
+        'ui',
+        _('Restore default font size'),
+    ),
+
     'increase_number_of_columns': desc(
         v"['Ctrl+]']",
         'ui',

src/pyj/read_book/view.pyj

@@ -26,7 +26,7 @@
 from read_book.open_book import add_book_to_recently_viewed
 from read_book.overlay import Overlay
 from read_book.prefs.colors import resolve_color_scheme
-from read_book.prefs.font_size import change_font_size_by
+from read_book.prefs.font_size import change_font_size_by, restore_default_font_size
 from read_book.prefs.fonts import current_zoom_step_size
 from read_book.prefs.head_foot import render_head_foot
 from read_book.prefs.scrolling import (
@@ -521,6 +521,8 @@ def on_handle_shortcut(self, data):
             self.bump_font_size({'increase': True})
         elif data.name is 'decrease_font_size':
             self.bump_font_size({'increase': False})
+        elif data.name is 'default_font_size':
+            restore_default_font_size()
         elif data.name is 'toggle_full_screen':
             ui_operations.toggle_full_screen()
         elif data.name is 'toggle_reference_mode':