GUI: Add Tab cycling to  TabWidget

Tab and Shift-Tab can now cycle between each Tab of the Edit Game menu. Changes occurred in dialog.cpp, tab.h, and tab.cpp.
GUI: Update tab.cpp
GUI: Corrected return before bracket.

The return before bracket has been fixed.
GUI: Changes to handleKeyDown to remove FIXME

Changes to handleKeyDown are made to make it more closely resemble above means of iterating through widgets on screen.
GUI: Fixes to code formatting

Fixes to issues with white space before and after operators applied.
GUI: Check if button is of type TabWidget

Changes to code for checking for Tab keys, so it only handles the key down state if the widget is of type TabWidget.
GUI: Correct white space when casting

White space when casting is no longer present between variables being cast.
Merge branch 'master' of https://github.com/Zerophase/scummvm