schema: Use only SQL single quotes for strings

New versions of sqlite no longer accept double
quotes for string literals.

On FreeBSD using sqlite 3.45.1 mixxx is unable
to start, even with an empty database, because
the migrations cannot be performed.

res/schema.xml

@@ -150,7 +150,7 @@ reapplying those migrations.
       Add file type column.
     </description>
     <sql>
-      ALTER TABLE library ADD COLUMN filetype varchar(8) DEFAULT "?";
+      ALTER TABLE library ADD COLUMN filetype varchar(8) DEFAULT '?';
     </sql>
   </revision>
   <revision version="5">
@@ -176,15 +176,15 @@ reapplying those migrations.
     <sql>
       ALTER TABLE library ADD COLUMN timesplayed INTEGER DEFAULT 0;
       ALTER TABLE library ADD COLUMN rating INTEGER DEFAULT 0;
-      ALTER TABLE library ADD COLUMN key varchar(8) DEFAULT "";
+      ALTER TABLE library ADD COLUMN key varchar(8) DEFAULT '';
       UPDATE library SET timesplayed = played;
       UPDATE library SET played = 0;
 
-      DELETE FROM settings WHERE name="mixxx.db.model.library.header_state";
-      DELETE FROM settings WHERE name="mixxx.db.model.playlist.header_state";
-      DELETE FROM settings WHERE name="mixxx.db.model.crate.header_state";
-      DELETE FROM settings WHERE name="mixxx.db.model.prepare.header_state";
-      DELETE FROM settings WHERE name="mixxx.db.model.missing.header_state";
+      DELETE FROM settings WHERE name='mixxx.db.model.library.header_state';
+      DELETE FROM settings WHERE name='mixxx.db.model.playlist.header_state';
+      DELETE FROM settings WHERE name='mixxx.db.model.crate.header_state';
+      DELETE FROM settings WHERE name='mixxx.db.model.prepare.header_state';
+      DELETE FROM settings WHERE name='mixxx.db.model.missing.header_state';
     </sql>
   </revision>
   <revision version="8" min_compatible="3">
@@ -305,7 +305,7 @@ reapplying those migrations.
       Add composer column to library table.
     </description>
     <sql>
-      ALTER TABLE library ADD COLUMN composer varchar(64) DEFAULT "";
+      ALTER TABLE library ADD COLUMN composer varchar(64) DEFAULT '';
     </sql>
   </revision>
   <revision version="15" min_compatible="3">
@@ -375,17 +375,17 @@ reapplying those migrations.
       Add grouping and album_artist column to library table.
     </description>
     <sql>
-      ALTER TABLE Library ADD COLUMN grouping TEXT DEFAULT "";
-      ALTER TABLE Library ADD COLUMN album_artist TEXT DEFAULT "";
+      ALTER TABLE Library ADD COLUMN grouping TEXT DEFAULT '';
+      ALTER TABLE Library ADD COLUMN album_artist TEXT DEFAULT '';
     </sql>
   </revision>
   <revision version="22" min_compatible="3">
     <description>
       Add grouping and album_artist column to itunes_library table.
     </description>
     <sql>
-      ALTER TABLE itunes_library ADD COLUMN grouping TEXT DEFAULT "";
-      ALTER TABLE itunes_library ADD COLUMN album_artist TEXT DEFAULT "";
+      ALTER TABLE itunes_library ADD COLUMN grouping TEXT DEFAULT '';
+      ALTER TABLE itunes_library ADD COLUMN album_artist TEXT DEFAULT '';
     </sql>
   </revision>
   <revision version="23" min_compatible="3">
@@ -406,7 +406,7 @@ reapplying those migrations.
     <sql>
       ALTER TABLE library ADD COLUMN coverart_source INTEGER DEFAULT 0;
       ALTER TABLE library ADD COLUMN coverart_type INTEGER DEFAULT 0;
-      ALTER TABLE library ADD COLUMN coverart_location TEXT DEFAULT "";
+      ALTER TABLE library ADD COLUMN coverart_location TEXT DEFAULT '';
       ALTER TABLE library ADD COLUMN coverart_hash INTEGER DEFAULT 0;
     </sql>
   </revision>