Adds confirmation dialog and help text to server and channel close.

In order to improve the UX this commit adds a confirmation dialog when
closing the connection to the server. It also adds a 'title'-tag over
the close-icon (an 'x') informing the user that pressing this button
will result in either leaving network or channel.
Merge pull request #293 from hermansc/close-confirmation-dialog

Adds confirmation dialog and help text to server and channel close.