UModel:
- added "About" dialog
- displaying git revision number in "-version" output, "About" dialog and crash reports
- added more menu items in "Help"
- UIBaseDialog::SetMainWindow() now called when SDL window created, not when "Open package" dialog displayed

UI:
- added UIHorizontalLine kind of separator
- added "hyperlink" menu item (executes any command)

Tools:
- build.sh now requests revision number from git and generates/updates UmodelTool/Version.h