Began GUI work

- Created a welcome screen, nothing is implemented there yet
- Created an entry point to the entire GUI (MediaStoreUI.java)
- Included into the project an open source helper class, GBC to help
with GridBayLayouts
- Included a look and feel theme called substance
- Modified the build files to include the substance JARs into a single
file (dist/mediastore_full.jar), it should launch correctly when
double-clicked or run from console