Fix slow library search on sqlite 3.8.

(cherry picked from commit 56dade25981da1656cbb68447e8518529e229978)

Conflicts:
	src/library/libraryquery.cpp
Add explanatory comment to last commit.

(cherry picked from commit 5fffbbc6898dc8373d6fd5bf28d4cd79d6f089d5)
Fix compiling 1.2 branch on GCC 4.9
Bump 1.2 release version
Bump changelog for 1.2.3

(cherry picked from commit 2e61ef9b178b20652590832b7d36038c73d0bbf5)
Add missing header for syscall().

This is cunningly not defined in syscall.h

(cherry picked from commit 3df3a5b645737603af6bda844db04c431b9c8064)
Add missing header for _exit()

Fixes #4188

(cherry picked from commit acfc7e6d2196f9ecb080b5405cd78066734bbc39)
Add missing include for symlink()

(cherry picked from commit 7671108aa8d02658cd35cacf4337507cc80ea03a)
Bump installed dependency DLL versions.

(cherry picked from commit 49c2fe8de11e06838feea34afa20aac4db187cbe)
Add new DLLs to windows install.

(cherry picked from commit 8c2f927bc22c32502d3a0673e9bc6c768543d31f)
Install libffi DLL.

(cherry picked from commit 1c3cee87192496b9439596ca50526bf14f760a95)
Add more things to the 1.2.3 changelog
(cherry picked from commit e8a0c1cd9f49943606e558331cd097ac888c9f70)
Add test for MusicBrainzClient.
Parse discid xml from MusicBrainz correctly. Fixes #4025.
Reset the model during init of CddaDevice

This ensures that the track list in the device tab is correctly updated when a
disc is replaced with one that doesn't exist in the MusicBrainz database.
Fix dbz possibility in moodbar

Took me forever to find this. Randomly happens when changing tracks.
Merge pull request #4376 from TheUbuntuGuy/master

Fix dbz possibility in moodbar
Add persistent disk cache for library pixmaps

I'll reference #4379

When viewing the library with album covers visible, the covers load very slowly the first time
as they have to wait on the tagreaders. If I scroll down the library, it takes minutes for the
tagreader to catch up. The nice thing is that the pixmaps are cached. However, once
Clementine is restarted, the whole process has to happen again.

This patch adds a persistent disk cache in the form of a QNetworkDiskCache to store the
pixmaps on disk and load them into the QPixmapCache as required.

I've noted literally night and day performance improvements, not only when scrolling through the library.
There is much better interface responsiveness when searching, and I no longer see the no_cover_icon
temporarily anymore.
Optimizations and style changes
More style changes