Add empty persistent panel between HUD and tabs

[robertlong13]

This adds an empty, zero-sized panel between the HUD and the tabs. This allows for the creation of plugins that add buttons or readouts that are always visible, and don't require clicking around through tabs.

As an example of how this could be useful, I have included an example .cs plugin in this pull request (disabled by default), similar to tabActionsSimple, with three mode switch buttons. The three modes can be changed to anything by simply tweaking three lines in the .cs file without having to rebuild anything.