Fix Jira issue #ROOT-6191: TBrowser and canvas editor

Embed the Pad Editor in the left tab of the browser instead of in the canvas itself, as suggested by Axel in the Jira issue #ROOT-6191
Fix Jira issue #ROOT-6221: Text editor does not allow negative positions

Apply patch kindly sent by Matthew Strait
Unanamed macros not detecting leading and trailing comments. (ROOT-5930 and ROOT-4791).
Multiple line of leading comments. (ROOT-5930 and ROOT-4791).
Only complain about inspecting std classes for persistent members.
Give libCling an extension.