Revert to using a QPlainText for extra css. Apparently the more sophisticated editor was causing crashes for some people. Fixes #1557147 [2.53 crashes in Qt5Gui.dll when starting conversions](https://bugs.launchpad.net/calibre/+bug/1557147)

src/calibre/gui2/convert/__init__.py

@@ -215,10 +215,10 @@ class Widget(QWidget):
             return
         if isinstance(g, (QSpinBox, QDoubleSpinBox)):
             g.setValue(val)
-        elif isinstance(g, (QLineEdit, QTextEdit)):
+        elif isinstance(g, (QLineEdit, QTextEdit, QPlainTextEdit)):
             if not val:
                 val = ''
-            getattr(g, 'setPlainText', g.setText)(val)
+            getattr(g, 'setPlainText', getattr(g, 'setText', None))(val)
             getattr(g, 'setCursorPosition', lambda x: x)(0)
         elif isinstance(g, QFontComboBox):
             g.setCurrentFont(QFont(val or ''))

src/calibre/gui2/convert/look_and_feel.py

@@ -8,7 +8,7 @@ __docformat__ = 'restructuredtext en'
 
 import json
 
-from PyQt5.Qt import Qt, QSize
+from PyQt5.Qt import Qt
 
 from calibre.gui2.convert.look_and_feel_ui import Ui_Form
 from calibre.gui2.convert import Widget
@@ -63,7 +63,6 @@ class LookAndFeelWidget(Widget, Ui_Form):
         self.opt_unsmarten_punctuation.stateChanged.connect(
                 lambda state: state != Qt.Unchecked and
                 self.opt_smarten_punctuation.setCheckState(Qt.Unchecked))
-        self.opt_extra_css.size_hint = QSize(400, 300)
 
     def get_value_handler(self, g):
         if g is self.opt_change_justification:
@@ -107,9 +106,6 @@ class LookAndFeelWidget(Widget, Ui_Form):
                     w.setChecked(False)
             self.filter_css_others.setText(', '.join(items))
             return True
-        if g is self.opt_extra_css:
-            g.load_text(val or '', 'css')
-            return True
         if g is self.opt_transform_css_rules:
             g.rules = json.loads(val) if val else []
             return True

src/calibre/gui2/convert/look_and_feel.ui

@@ -374,7 +374,7 @@
          </property>
          <layout class="QGridLayout" name="gridLayout_4">
           <item row="0" column="0">
-           <widget class="TextEdit" name="opt_extra_css"/>
+           <widget class="QPlainTextEdit" name="opt_extra_css"/>
           </item>
          </layout>
         </widget>
@@ -515,11 +515,6 @@
    <extends>QWidget</extends>
    <header>calibre/gui2/font_family_chooser.h</header>
   </customwidget>
-  <customwidget>
-   <class>TextEdit</class>
-   <extends>QPlainTextEdit</extends>
-   <header>calibre/gui2/tweak_book/editor/text.h</header>
-  </customwidget>
   <customwidget>
    <class>RulesWidget</class>
    <extends>QWidget</extends>