Adjust wrt-installer AppWidgetParser to current specification

Adjust wrt-installer AppWidgetParser to current specification

[Issue#] LINUXWRT-322
[Problem] N/A
[Cause] N/A
[Solution] adjusted to specification http://slp-info.sec.samsung.net/confluence/display/BROWSER/Tizen+Web+Runtime+Core+Specification+2.1#TizenWebRuntimeCoreSpecification21-Web%C2%A0AppWidgetConfigurationExtensions
[Verification] Build wrt-installer repository
Tests located in test/wrt repository in wrt-webapp-widget-tests subdir shouldn't be installed:
app-widget-icon-null.wgt
app-widget-content-null.wgt
widget-app-box-size-null.wgt
app-widget-box-size-invalid.wgt

Change-Id: I70460106b2983a360c800b084f41f942a18201c8