[WebInspector] Use TabWidget::currentChanged(int) signal and check for invalid tab

TabWidget::currentChanged(int index) can be emited with  index == -1,
which means there are no tabs in tabbar. This may lead to crash after
accessing current webview (null)
[NetworkCache] Fix path for network cache
[Toolbars] Fixed toggle show Status Bar action

Closes #1259
[CreatingWindow] Don't disable updates when creating browser windows