Add an option to control how completion works when editing authors/tags/series/etc.

By default, when completing items, calibre will show you all the
candidates that start with the text you have already typed. You can
instead have it show all candidates that contain the text you have
already typed. To do this, go to Preferences->Tweaks->Completion mode.
For example, if you type asi it will now match both Asimov and Quasimodo,
whereas the default behavior would match only Asimov. Fixes #1264304 [[Enhancement] search within string when tagging](https://bugs.launchpad.net/calibre/+bug/1264304)

resources/default_tweaks.py

@@ -523,6 +523,14 @@
 # your library and your personal editing style.
 preselect_first_completion = False
 
+#: Completion mode when editing authors/tags/series/etc.
+# By default, when completing items, calibre will show you all the candidates
+# that start with the text you have already typed. You can instead have it show
+# all candidates that contain the text you have already typed. To do this, set
+# completion_mode to 'contains'. For example, if you type asi it will match both
+# Asimov and Quasimodo, whereas the default behavior would match only Asimov.
+completion_mode = 'prefix'
+
 #: Recognize numbers inside text when sorting
 # This means that when sorting on text fields like title the text "Book 2"
 # will sort before the text "Book 100". If you want this behavior, set

src/calibre/gui2/complete2.py

@@ -14,11 +14,14 @@
         QApplication, QListView, QPoint, QModelIndex, QFont, QFontInfo)
 
 from calibre.constants import isosx, get_osx_version
-from calibre.utils.icu import sort_key, primary_startswith
+from calibre.utils.icu import sort_key, primary_startswith, primary_icu_find
 from calibre.gui2 import NONE
 from calibre.gui2.widgets import EnComboBox, LineEditECM
 from calibre.utils.config import tweaks
 
+def containsq(x, prefix):
+    return primary_icu_find(prefix, x)[0] != -1
+
 class CompleteModel(QAbstractListModel):  # {{{
 
     def __init__(self, parent=None):
@@ -45,8 +48,8 @@ def set_completion_prefix(self, prefix):
             return
         subset = prefix.startswith(old_prefix)
         universe = self.current_items if subset else self.all_items
-        self.current_items = tuple(x for x in universe if primary_startswith(x,
-            prefix))
+        func = primary_startswith if tweaks['completion_mode'] == 'prefix' else containsq
+        self.current_items = tuple(x for x in universe if func(x, prefix))
         self.reset()
 
     def rowCount(self, *args):

src/calibre/utils/icu.py

@@ -247,6 +247,9 @@ def primary_find(pat, src):
     if _icu_not_ok:
         from calibre.utils.filenames import ascii_text
         return py_find(ascii_text(pat), ascii_text(src))
+    return primary_icu_find(pat, src)
+
+def primary_icu_find(pat, src):
     try:
         return icu_find(_primary_collator, pat, src)
     except AttributeError: