Set MainPanel to be clamped to the screen.

Set MainPanel to be clamped to the screen.
Lowercased all of the MainPanel.filter_menu members (general, obtain, etc)
Moved Generic_MakeCheckButton() to just after the creation of MainPanel.filter_menu
Moved creation of MainPanel.filter_menu.general and its check-buttons to the main file body from addon:InitializeFrame()
All of the CheckButtons for the General filter menu have been moved out of the global namespace and into MainPanel.filter_menu.general - they also no longer inherit from UICheckButtonTemplate.
In Generic_MakecheckButton(): Create a FontString and Normal/Pushed/Highlight/Disabled/Checked textures for the CheckButton, since it will no longer inherit from UICheckButtonTemplate.
Updated references to the General filter checkbuttons in FilterValueMap.
Re-wrote the OnClick script for MainPanel.filter_menu.general.class_toggle for clarity/efficiency.