- fix memory leak in find-usages
- do not retain snapshot in search history
- when an editor is invisible for more than 2 minutes, release the
  backing snapshot

Retaining snapshots will retain their documents, and if done for too
long, the memory consumption might grow. This is especially the case
when switching to a different kit (Qt version): in that case, the new
versions of headers will be indexed, while the old ones stay around.

Task-number: QTCREATORBUG-5583
Task-number: QTCREATORBUG-7645
Task-number: QTCREATORBUG-9842

Change-Id: I045eda1565e0a3fa702baeffaab9c12662f90289
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

Task-number: QTCREATORBUG-10280

Change-Id: Ibc6ffeedb97a31a35a2d1f00625506f844692e16
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

... when invoked on a macro that is defined inside that dialog.

Change-Id: I419c6675d70cfb8dd174f6a33f502b4c584baa3a
Reviewed-by: David Schulz <david.schulz@digia.com>

Task-number: QTCREATORBUG-10282

Change-Id: Ia58359f774c72260956caecd5f63d44b612d1451
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

Change-Id: Ic3578bd98c2f1aefcaef61f5b993dc9a67ca5a77
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

The UI side will come in another commit.

Change-Id: I1038a438587fe4cef70408f7f627d08837fc192d
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

Task-number: QTCREATORBUG-476

Change-Id: I82ed92acdcda551d2c6a9ca221832ac20117a08f
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

If two files from different (sub-)projects include the same header file,
and the defined macros differ for both files, the header file will be
parsed with only the appropriate macros for the including file.

Task-number: QTCREATORBUG-9802
Task-number: QTCREATORBUG-1249

Change-Id: I560490afa287b3bb1e863bce1bb4f57af36ad56e
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>