[client] Show a user icon.
[client][Servers] Show icons on buttons.
[client][Actions] Show icons on buttons.
[client][Events] Show icons on buttons.
[client][Users] Show icons on buttons.
[server][FaceRest] Include a DataQueryContext instance in RestJob.
[server][FaceRest] Improve perfomance by sharing a DataQueryContext instance.
[server][DBClientHatohol] Add a new version of HostResourceQueryOption's constructor().
[server][DBClientHatohol] Check the used count.
[server][DBClientHatohol] Check the used count.
[server][DBClientHatohol] Add a new version of TriggersQueryOption's constructor().
[server][testDBClientHatohol] Avoid a crash of the test.

When we use SYSTEM_ID_USER, OperationPrivilege doesn't access the config
DB. So the following problem can be avoided.

=========
https://travis-ci.org/project-hatohol/hatohol/jobs/20510396

  testDBClientHatohol::test_constructorDataQueryContext:[ERR] <DBAgentMySQL.cc:481> Failed to connect to MySQL: hatohol: (error: 1044, Domain ID: 0x30) Access denied for user 'hatohol'@'localhost' to database 'hatohol'
terminate called after throwing an instance of 'HatoholException'
  what():  <DBAgentMySQL.cc:90> Failed to connect to MySQL: hatohol: Access denied for user 'hatohol'@'localhost' to database 'hatohol'
!
===============================================================================
Crash: testDBClientHatohol::test_constructorDataQueryContext
===============================================================================
: (0.032216)
[server][FaceRest::RestJob] Rename a meber for improving readablity.

dqCtxPtr ->
dataQueryContextPtr
[client][FaceRest] Pass a DataQueryContext instance to improve performance.
[server][DBClientHatohol] Add a new version of EventsQueryOption's constructor().
[server][testDBClientHatohol] Check the value set in the constructor.
[server][FaceRest] Use a DataQueryContext instance for improving performance.
[server][DBClientHatohol] Add a new version of EventsQueryOption's constructor().
[client][FaceRest] Pass a DataQueryContext instance to improve performance.
[server][DBClientHatohol] Add a new version of ItemsQueryOption's constructor().