UI changes to Main application

Moved Game Generator and Preview tool under a new tab Game Generation
Collection
Extended Observable for Model classes. Deleting the common files from
game Engine package
Corrected name mismatch for behaviors and rewards
Included the missing propOrder = "rewards"
TestGame1 with new spec
Changed the behavior to endgame
Changes to GameElementType.java and Prop.java. Color and text information has been shifted from GameElementType to Prop. Reward has also been removed from GameElementType, per feedback from the professor.
Merge branch 'master' of https://github.com/9124SAVOIAD/SimSys
Changes to GameElementType.java and Prop.java. Color and text information has been shifted from GameElementType to Prop. Reward has also been removed from GameElementType, per feedback from the professor.
<game> tag error fix
UI changes to main application window

Added inactive tabs for tools that are yet to developed.
Re factored code to have proper naming of elements in Game tree
displayed in the side panel of preview tool.
Reorganizing Preview tool

ErrorChecker - contains the error checking module
Errors - contains list of errors and their severities
View - contains the user interface module of preview of tool
Reorganizing Preview Tool
Reorganizing Preview Tool

Renamed View package to UserInterface package
Moved 4 asset selection window classes to new package
'AssetSelectionWIndows' under UserInterface package
Removing moved classes from old package
Merging Default Changes
Changes to Item, Layout, MultipleChoiceItem, Option, QuizChallenge, Stem, StemQuestion, StemText to make these classes more in line with the game specification as written.