Improvements for editing on tab controls

[altenbach]

My front panels are typically large tab controls so most of the FP design and editing takes place on tabs.

In this scenario we lose significant usability, for example:

• We cannot double click on empty space to create a free label
• We cannot see the alignment grid.
• ...

Proposal:

• The background alignment grid should continue across tab controls and other containers (e.g. clusters) in edit mode.
• double-clicking inside an empty area on a tab should start a free label operation, same as on the plain background. (only clicking near the border should "find" the terminal instead).